ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 81 
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 Amphzoxus occurred in a radially 
symmetrical manner, the entire border 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 іп 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 case 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 723°. 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 іп 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 еве divisions take place as rapidly 
11 JOURN. А. N. 8. PHILA., VOL. хш. 
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