No. 3-] THE GASTRULATION OF AMPHIOXUS. 593 



Gastrulamundes wahrend der Gastrulaschliessung unverandert 

 bleibt. Dies ist aber auch nicht der Fall, und Hatschek's 

 eigene Abbildungen sprechen nicht zu Gunsten dieser An- 

 nahme." ^ 



Our view of the method of closing of the blastopore agrees 

 essentially with Lwoff's, that is, that the blastopore closes 

 equally from all sides, and that the gastrula axis corresponds 

 more or less exactly with the longitudinal axis of the embryo. 

 On the other hand, we have tried to show that there is no 

 necessity for supposing that the cells outside the blastopore 

 on the dorsal side turn in during the period of closure of the 

 blastopore. It seems more probable that the advance of the 

 dorsal lip takes place in the same way as that of the lateral 

 and ventral lips, and even Lwoff does not suppose the latter to 

 advance as the result of the inturning of cells. 



Wilson made the important observation that " the cleavage 

 pore, which marks the lower pole of the blastula, sometimes 

 persists up to a stage as late as the gastrula shown in Hat- 

 schek's Figs. 26 and 27. In all such cases I examined, it lay 

 exactly at the central point of the dome — a fact that shows 

 that the invagination is primarily symmetrical, as originally 

 described by Kowalevski." 



Wilson also states, in contradiction to Lwoff's statement, that 

 the entoblastic cells (macromeres) " show numerous conspicu- 

 ous mitoses, and in every part of the entoblastic plate." ^ 



Klaatsch has given a few figures, mainly optical sections of 

 preserved embryos of Amphioxus. He looked for concrescence 

 but failed to find any evidence of it. In regard to the orienta- 

 tion of the embryo he found the closure of the blastopore as 

 described by Kowalevski, " von vorn herein fast genauer aboral 

 und ganz geringe Neigung zur dorsalen Seite hin." ^ 



Sobotta's account follows Hatschek's very closely, and adds 

 little that is new. He states that no distinction exists between 

 the dorsal and ventral wall of the archenteron, during the early 

 gastrula stages, as Lwoff maintained. The explanation of this 

 lies, no doubt, in the preserving fluids that were used. So- 

 botta states that in the eggs from Naples the blastopore 



1 Lwoff, p. 14. 2 Wilson, p. 586. ^ Klaatsch, p. 229. 



