BLASTOGENESIS AND MORPHOGENESIS
153
Animals capable of budding have a simple and primitive organization,
as can be seen in the acoelomates, sponges, and coelenterates; the annelids and coelomates with a basic primitive organization; and even some
highly evolved animals, simplified by regressive evolution, such as turbellarians, bryozoans, and tunicates.
In blastogenic organisms, growth does not stop, however, when the
specific structure being formed is completed. It continues in a specific
area, the budding zone or blastogenic
zone.
Everything appears as if, in budding organisms, whether simple or
secondarily simplified, the correlating action which assures coordination
is less powerful than in animals which are more highly evolved, more
complex, more specialized, or more highly differentiated. The effect of
growth appears to be the provocation of a rupture in coordination, a
disruption of physiological correlations, to set free in the blastogenic zone
some somatic cells from a coercive and coordinating influence which insures the morphological and physiological unity of the animal. Child
(1924) has called this "physiologic isolation." Up to the present time, the
significance of physiological isolation has not been precisely stated, nor
actually even studied. Of what does this localized physiological state
consist, by means of which somatic tissues again become ontogenic? This
is one of the most important problems of general biology since it is concerned with the reversibility of living processes, a question which will be
considered below.
Blastogenesis, thus, is localized and is the result of growth. In the inner
layer of mesenchyme of freshwater sponges, gemmules are formed. Their
number increases simultaneously and proportionately as the sponge becomes larger and thicker.
In the freshwater hydra, the blastogenic zone is located at the base of
the body column, a little above the peduncle. In a well fed Hydra
fusca,
the blastogenic zone usually has several buds (Fig. 1). When the sequence of their appearance is followed, is noted that the best developed
buds, those that complete their organogenesis in a small hydra, are situated at the level of the insertion of the peduncle. They are about to be
detached. The youngest buds, visible as swelling of the body wall, occupy
the superior region of the blastogenic zone. Between the oldest buds,
ready to be liberated, and the youngest ones, which are still only slight
swellings, are tiers of buds in intermediate stages of development. They
are not arranged at random in the blastogenic zone, but in alternate
rows, in quincunx, in a helical path, three buds occupying one turn. In
this position, they descend from the superior region of the blastogenic
153
Animals capable of budding have a simple and primitive organization,
as can be seen in the acoelomates, sponges, and coelenterates; the annelids and coelomates with a basic primitive organization; and even some
highly evolved animals, simplified by regressive evolution, such as turbellarians, bryozoans, and tunicates.
In blastogenic organisms, growth does not stop, however, when the
specific structure being formed is completed. It continues in a specific
area, the budding zone or blastogenic
zone.
Everything appears as if, in budding organisms, whether simple or
secondarily simplified, the correlating action which assures coordination
is less powerful than in animals which are more highly evolved, more
complex, more specialized, or more highly differentiated. The effect of
growth appears to be the provocation of a rupture in coordination, a
disruption of physiological correlations, to set free in the blastogenic zone
some somatic cells from a coercive and coordinating influence which insures the morphological and physiological unity of the animal. Child
(1924) has called this "physiologic isolation." Up to the present time, the
significance of physiological isolation has not been precisely stated, nor
actually even studied. Of what does this localized physiological state
consist, by means of which somatic tissues again become ontogenic? This
is one of the most important problems of general biology since it is concerned with the reversibility of living processes, a question which will be
considered below.
Blastogenesis, thus, is localized and is the result of growth. In the inner
layer of mesenchyme of freshwater sponges, gemmules are formed. Their
number increases simultaneously and proportionately as the sponge becomes larger and thicker.
In the freshwater hydra, the blastogenic zone is located at the base of
the body column, a little above the peduncle. In a well fed Hydra
fusca,
the blastogenic zone usually has several buds (Fig. 1). When the sequence of their appearance is followed, is noted that the best developed
buds, those that complete their organogenesis in a small hydra, are situated at the level of the insertion of the peduncle. They are about to be
detached. The youngest buds, visible as swelling of the body wall, occupy
the superior region of the blastogenic zone. Between the oldest buds,
ready to be liberated, and the youngest ones, which are still only slight
swellings, are tiers of buds in intermediate stages of development. They
are not arranged at random in the blastogenic zone, but in alternate
rows, in quincunx, in a helical path, three buds occupying one turn. In
this position, they descend from the superior region of the blastogenic