342 TUNICATA. 



embryo. Gastrulation in the Ascidians has consequently frequently 

 been described as transitional between the typical epibolic and 

 embolic conditions. 



The gasirula-stage that thus arises (Fig. 151, C) is saucer-shaped. 

 The arched surface of the embryo is covered with the small ectoderm- 

 cells, while the flattened side of the body is occupied by the large, 

 round blastopore. This side is said to change into the later dorsal 

 surface of the embryo, while the arched side becomes the ventral 

 surface. 



If we take as the principal axis of this gastrula-stage (Fig. 152 A) that 

 which connects the animal with the vegetative pole of the egg in the first 

 stages of cleavage (a-b), we find that this axis passes through the apex of the 

 arched portion on the one hand and through the centre of the blastopore on 

 the other. The future longitudinal axis of the body, on the contrary, would 

 lie at right angles to this principal axis, since, according to all observers, the 

 blastopore corresponds to the later dorsal surface. We should then have a 

 condition differing from that of other Bilateralia, in which the primary axis 

 corresponds approximately to the longitudinal axis of the adult. It seems 

 probable that the blastopore shifts secondarily from the vegetative pole of the 

 embryo, at which the posterior end of the body now develops to a position on 

 the dorsal surface. According to this orientation, the dorsal and ventral 

 surfaces would, as in the first ontogenetic stages of most of the Bilateralia, 

 occupy a meridional position. After what has been stated above, we may 

 conjecture that, in Ascidians, the displacement of the blastopore to the dorsal 

 side of the body is first brought about by shifting caused by growth in the 

 later gastrula-stages such as is indicated by the orientation adopted in Fig. 

 152 A-C. A comparison with the ontogeny of Amphioxus to a certain extent 

 supports this supposition (Chap. XXXVI.). According to this view, the 

 orientation of the cleavage-stages given by some authors, in which the animal 

 pole of the body is said to be related to the future ventral half and the 

 vegetative to the later dorsal half, does not appear altogether suitable, and 

 can only be admitted with certain reservations.* 



Bilateral symmetry can be recognised in the gastrula-stage, even 

 at the first, by the distribution of the cells. In later stages this 



* [Castle, in his work on Ciona, supports the latter view, that is, he finds 

 that the vegetative pole with the entoderm and blastopore is always dorsal in 

 position. The apparent shifting of the blastopore is due to the fact that it 

 closes more rapidly from the anterior margin and from the sides than from 

 behind. Consequently it comes to lie in the posterior portion of the dorsal 

 surface of the embryo. He is therefore in agreement with Samassa (No. 

 XXXII.) when he states that there is no rotation of the axes during gastrula- 

 tion as conjectured on theoretical grounds by Korschelt and Heider, but 

 that the primitive axis corresponds with the vertical axis of the larva and 

 the longitudinal axis is at right angles to this. According to this inter- 

 pretation, the orientation of Fig. 152 is incorrect, since the blastopore, in 

 each of the three stages, is represented on a different surface, whereas it is 

 always dorsal in its position. — Ed.] 



