EARLY DEVELOPMENT OF THE SEA URCHIN
127
prevent the formation of a multicellular blastula'. At the early blastula
stage, on the other hand, nuclear control begins and the period between
the early- and the mid-blastula stage appears to control development
until the late gastrula stage. Then again a period follows from the late to
the mesenchyme blastula stage when the nuclei do not appear to exert
any control on development and this is shown again by the fact that no
matter when, within this interval, the embryo is irradiated, development will be arrested at the mid-gastrula stage. At the mesenchyme
blastula stage the nuclei have a further period of activity and control
further development. These observations, if confirmed, would be of
great importance as they prove that the nuclear influence on development operates in a wave-like fashion, periods of greater activity
alternating with periods of low, if any, influence on morphogenesis. Of
course the validity of these conclusions rests on the assumption that
under the conditions of the author's experiments the irradiation caused
no damage to the cytoplasm or that the cytoplasmic damage was
inconsequential as compared to that caused to the nucleus. However,
the development of anucleate eggs of Arbacia irradiated with a dose of
17 to 60 χ 10
3 r and then fertilized with normal sperm has shown a
significant mitotic delay (Rustad, 1961). Although, admittedly, the
significance of the mitotic delay observed under the above conditions
may be doubtful with regard to morphogenesis, the writers feel that in
view of the importance of Neyfakh's results, a more thorough investigation of the radio-sensitivity of the cytoplasm in various stages of
development would be highly desirable.
More recent experiments by Neyfakh (1961) show that in the eggs of
the teleostean fish, Misgurnus fossilis (as observed in other fish eggs
also), oxygen consumption does not begin to increase until the blastula
stage but the activity of cytochrome oxidase increases from the very
beginning of development. Now, an irradiation applied during the first
5 to 6 minutes following fertilization completely inhibits the activity
increase of cytochrome oxidase, whereas if applied after the 7th to 8th
minute it is quite ineffective. This is interpreted as evidence that the
nuclear control of respiration is established immediately following
fertilization.
This latter observation calls to mind the older one by Glass and Plaine
(1950). In Drosophila the semidominant gene erupt (er), causing the
appearance of a large mass of non-faceted material in the central part of
one or both eyes, is counteracted by another semidominant gene,
suppressor of erupt (su-er). These two genes belong to different linkage
groups and undergo Mendelian segregation. An irradiation of 1,000 r
given to the embryos between the 1st and the 24th hour of development
causes the inactivation of su-er and the phenotype will therefore be er.
127
prevent the formation of a multicellular blastula'. At the early blastula
stage, on the other hand, nuclear control begins and the period between
the early- and the mid-blastula stage appears to control development
until the late gastrula stage. Then again a period follows from the late to
the mesenchyme blastula stage when the nuclei do not appear to exert
any control on development and this is shown again by the fact that no
matter when, within this interval, the embryo is irradiated, development will be arrested at the mid-gastrula stage. At the mesenchyme
blastula stage the nuclei have a further period of activity and control
further development. These observations, if confirmed, would be of
great importance as they prove that the nuclear influence on development operates in a wave-like fashion, periods of greater activity
alternating with periods of low, if any, influence on morphogenesis. Of
course the validity of these conclusions rests on the assumption that
under the conditions of the author's experiments the irradiation caused
no damage to the cytoplasm or that the cytoplasmic damage was
inconsequential as compared to that caused to the nucleus. However,
the development of anucleate eggs of Arbacia irradiated with a dose of
17 to 60 χ 10
3 r and then fertilized with normal sperm has shown a
significant mitotic delay (Rustad, 1961). Although, admittedly, the
significance of the mitotic delay observed under the above conditions
may be doubtful with regard to morphogenesis, the writers feel that in
view of the importance of Neyfakh's results, a more thorough investigation of the radio-sensitivity of the cytoplasm in various stages of
development would be highly desirable.
More recent experiments by Neyfakh (1961) show that in the eggs of
the teleostean fish, Misgurnus fossilis (as observed in other fish eggs
also), oxygen consumption does not begin to increase until the blastula
stage but the activity of cytochrome oxidase increases from the very
beginning of development. Now, an irradiation applied during the first
5 to 6 minutes following fertilization completely inhibits the activity
increase of cytochrome oxidase, whereas if applied after the 7th to 8th
minute it is quite ineffective. This is interpreted as evidence that the
nuclear control of respiration is established immediately following
fertilization.
This latter observation calls to mind the older one by Glass and Plaine
(1950). In Drosophila the semidominant gene erupt (er), causing the
appearance of a large mass of non-faceted material in the central part of
one or both eyes, is counteracted by another semidominant gene,
suppressor of erupt (su-er). These two genes belong to different linkage
groups and undergo Mendelian segregation. An irradiation of 1,000 r
given to the embryos between the 1st and the 24th hour of development
causes the inactivation of su-er and the phenotype will therefore be er.