ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. SI 



that the posterior lip takes only a small part in the closure of the blastopore, except 

 in the final stages of that process. The posterior border of the blastopore is formed 

 of mesodermal cells derived from the crescent ; these cells are larger and more 

 rounded than the cells of the anterior border and are easily distinguishable by 

 their color and texture. In the closure of the blastopore they are rolled in at 

 the lateral margins but not at the hinder end. and owing to the large size of 

 these " myoblasts " the posterior portion of the blastopore is reduced to a longi- 

 tudinal groove. Finally, this groove is closed by growth from all sides, the pos- 

 terior lip growing more rapidly than the anterior one in the final stages of the 

 process. 



The earliest trace of the anterior lip appears just posterior to the chorda 

 cells, the endodermal cells here becoming depressed (fig. 134); at this stage the 

 chorda cells and the neural plate cells which lie just anterior to them are at the 

 same level, but in the posterior growth of this lip the chorda cells are rolled in so 

 that they form the inner, as the neural plate cells form the outer, layer of the ante- 

 rior (dorsal) lip. None of the neural plate cells and none of the ectodermal cells 

 are ever inrolled, the only cells which suffer this fate being the chorda cells and 

 the muscle cells (myoblasts). 



There has been much controversy as to the part played by the anterior and 

 the posterior lips in the closure of the blastopore in Amphioxus and the amphib- 

 ians. Kowalevsky supposed that the closure in Amphioxus occurred in a radially 

 symmetrical manner, the entire bolder of the blastopore growing equally ; Hatschek 

 thought that the growth of the anterior (dorsal) lip was the chief factor in the 

 closure; Lwoff, Klaatsch, Samassa, Morgan and Hazen agree in the main with 

 Kowalevsky. MacBride (1898) finds that in the final stages of closure the ventral 

 lip grows more rapidly than the dorsal. 



Among the amphibians, observation and experiment show that the over- 

 growth of the dorsal lip is greater than that of the ventral, but the relative 

 amount of growth of each lip is not certain. In early stages of closure the dorsal 

 lip is alone concerned, as is also the ease with ascidians ; in later stages growth 

 takes place from all sides. According to Pfliiger the dorsal lip sometimes moves 

 through an are of 180 in the case of the frog, according to Roux 170; Morgan 

 estimates this movement at 120, Kopsch at 75. and H. V. Wilson at 722. In 

 the ascidians there is no doubt whatever that the closure is due chiefly to the 

 growth of the dorsal lip, though owing to the changing shape of the embryo it 

 is difficult to estimate the angular amount of that growth. 



In Amphioxus and the ascidians the growth of the dorsal lip occurs as rapidly 

 in the middle as at the sides and there is therefore no indication of concres- 

 cence of lateral lips. At no stage during the closure of the blastopore in these 

 animals is there any indication whatever of such concrescence, either in the 

 form of a notch at the edge of the dorsal lip or of a seam along the middle of 

 the neural plate. In Cynthia and Ciona I have seen every division of the cells 

 of this lip up to an advanced stage and these divisions take place as rapidly 



n jouen a. x. > run. A., VOL. XIII. 



