CHAPTER
II
THE EMBRYOLOGY OF ASCIDIANS
G. REVERBERI
I. Introduction
55
II. The Biochemical Constituents of the Egg and Their Segregation During
Development
55
A.
The Constituents of the Centrifuged Egg
56
B.
The Constituents of the Normal (not Centrifuged) Egg
58
C.
The Segregation of the Egg Constitutents in Development
60
III. The Biological Significance of the Egg Constituents
68
A.
Quantitative Variations of Certain Egg Constituents During Development
68
B.
Specific Inhibition of Enzymatic Activity
71
C.
Heterospecific Hybridizations
77
IV. Egg Constituents and Morphogenesis
85
A.
The Organ-forming' Potencies of the Unsegmented Egg
85
B.
The'Mosaic'Character of Development
92
C.
The'Inductors'
94
Acknowledgements
99
References
v
99
I. Introduction
As a consequence of the numerous studies on Ascidian embryology
which have been carried out in recent years it is necessary to modify
some already accepted conclusions. It is the purpose of this review to
present the facts which justify the changes. These facts are arranged
under the headings: II. The biochemical constituents of the egg and
their segregation during development; III. Biological significance of
the egg constituents; IV. Egg constituents and morphogenesis.
II. The Biochemical Constituents of the Egg
and Their Segregation During Development
The Ascidian egg, when detached from the ovary, passes into the
oviduct, where it can remain for a long time; at this stage it does not
grow any further. It possesses all the necessary constituents for development; the germinal vesicle has collapsed, and the chromosomes are
situated at the equator of a small achromatic spindle which lies immediately under the cortex, at the animal pole. The egg can remain for a
55