322 



Embryogenesis: Progressive Differentiation 



■NERVE CORD 

 -CEREBRAL VESICLE AND SENSORY CELLS 



ADHESIVE PAPILLAE 



Fig. 121. Cleavage stages and prospective fates of specific regions of the egg of the tunicate Styela 

 {Cynthia) partita. In all illustrations the anterior end is to the left, the posterior end to the right. {A, B, 

 C, F, G and / from Conklin, '05a; D and E from Conklin, '05b.) A, Unfertilized egg showing germ.inal 

 vesicle (GV), central gray yolk (oblique hatching), and peripheral yellow protoplasm (dotted). B, Fertilized 

 egg showing accumulation of yellow protoplasm and clear protoplasm at the vegetal pole. C, Uncleaved, 

 fertilized egg with yellow protoplasm accumulated as a crescent across the posterior end of the egg. D, 2-cell 

 stage showing light gray crescent (vertical hatching) across the anterior end of the egg. E, 8-cell stage. 

 F, Animal half of 64-cell stage viewed from the animal pole; vertically-hatched cells represent prospective 

 cerebral vesicle. G, Vegetal half of 64-cell stage viewed from vegetal pole; vertically-hatched cells represent 

 prospective nerve cord; cells hatched vertically and horizontally represent prospective chorda; solid dots 

 represent prospective muscle cells; circles represent prospective mesench3Tne. H, Gastrulation stage; dia- 

 grammatic. /, Yoimg tadpole larva. /, Definitive tadpole larva; diagrammatic. 



half anterior to the yellow crescent contains 

 dark gray protoplasm. The first cleavage fur- 

 row passes through the mid-sagittal plane 

 (the plane of the paper in Figs. 121C, D), 

 bisecting the yellow crescent, and also bi- 

 secting a light gray crescent which has now 

 become visible around the anterior end of the 

 egg (Fig. 12125), extending slightly above 

 the equator. Thus there is in this egg shortly 

 after fertilization an early, sharp, and visible 

 localization of several indices of underlying 



differences in cytoplasmic composition. 

 Cleavage progressively segregates these re- 

 gions from one another until by the 64-cell 

 stage each cell is "pure" for cytoplasm of one 

 type only (Figs. 121E, F, G). The structures 

 of the tadpole larva derived from each group 

 of distinctively colored cells can be deter- 

 mined at a glance (Figs. \2\H, I, J). These 

 ooplasmic regions are sufficiently distinct 

 that every important step of their localization 

 and segregation can be followed in black and 



