68 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



cells .and form the anterior border of the blastopore belong to the dorsal hemi- 

 sphere ; the rest of this plate is composed of cells of the ventral hemisphere, 

 arranged in four rows of six each, and lying just in front of the chorda region ; 

 therefore in the 218-cell stage the nerve plate consists of forty cells arranged in six 

 transverse rows, each containing six cells except, the two most posterior rows which 

 contain eight each. The animal pole, with the polar bodies still attached, is still 

 situated back of the middle of the ventral face, six cell rows posterior to the 

 anterior edge of the neutral plate ; there are but five rows of cells of the ventral 

 hemisphere posterior to the animal pole, while there are ten such rows in front 

 of it. 



V. LATER DEVELOPMENT. 



My observations on the later stages of development agree in the main with 

 those of previous workers and particularly with those of Castle, who made a thorough 

 study of these stages by means of serial sections. It is true of these stages, how- 

 ever, as it is of the cleavage stages, that many topographical relations can be made 

 out more satisfactorily by a study of entire preparations. I have therefore 

 devoted especial attention to such preparations, and my observation, both on living 

 and on stained material, ai*e embodied in plates V and X. 



1. Closure of Blastopore. 



During the closure of the blastopore the embryo changes shape and at the 

 same time the egg axis is shifted. This stage is therefore an important one in 

 the orientation of the later stages. The gastrula is at first disk-shaped (fig. 134), it 

 then becomes saucer-shaped (fig. 136) and then cup-shaped (figs. 144, 145). During 

 this change as the embryo increases in depth it decreases in its other dimen- 

 sions so that it becomes more nearly spherical (fig. 145). The closure of the 

 blastopore takes place more rapidly from the anterior than from the posterior 

 side ; in fact after the general drawing together of the margins of the saucer- 

 shaped gastrula the posterior lip remains nearly stationary until the last stage in 

 the closure of the blastopore. 



Soon after the 218-cell stage the gastrula becomes elongated and egg-shaped, 

 the posterior end being somewhat narrower than the anterior. The anterior lip of 

 the blastopore continues to grow 'posteriorly while the lateral lips draw nearer 

 together; thus the blastopore becomes T-shaped (fig. 148), and finally, by the 

 further growth of the anterior lip, the anterior part of the blastopore, represented 

 by the bar of the T, is covered and the blastopore is reduced to a longitudinal 

 groove between the lateral lips (figs. 152, 153). In the growth of the lateral lips 

 they come to lie at a higher level than the anterior lip. and consequently as the 

 latter continues to. grow posteriorly, the former are tilted up at their anterior 

 ends until they become vertical in position. These lateral lips are at first formed 

 only of the muscle cells, but later the ectoderm cells completely overgrow them. 



