252 BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY. 



(b) Closure of the Blastopore. 



In Figure 78 (Plate XL) is shown a section parallel and slightly lat- 

 eral to the median plane of an embryo a little more advanced than any 

 thus far examined. It shows how the closure of the blastopore is 

 coming about. 



The ectoderm cells on the ventral surface are seen to be smaller 

 toward the anterior (left in the figure) than toward the posteiior end 

 of the embryo. They have evidently divided and passed into a later 

 generation than those more posteriorly situated. This has caused them 

 to spread over a greater surface, and has shoved the cells anterior to 

 them farther around on to the dorsal surface of the embryo. In the 

 anterior chorda cells, one of which is seen — in mitosis — in Figure 78 

 (a^-", a^'^^), division is nearly completed, the spindles standing about ver- 

 tically. By the continued overgowth of the anterior lip of the blastopore, 

 the more dorsally situated of the daughter cells in the chorda fundament, 

 e. g. a^-", are carried posterior to their sister cells, e. g. a^-"^^ (cf. Fig. 79), 

 and are finally entirely covered from sight by the nerve cells. They then 

 form a plate of eight cells lying in the dorsal wall of the archenteron 

 anterior to the blastopore. 



The endoderm cells of this embryo (Plate XI. Fig. 78) have under- 

 gone no new divisions since the 128-cell stage (Figs. 73-77), though 

 their nuclei have considerably increased in size, as is invariably the 

 case during the resting period. Moi'e lateral sections than the one 

 shown in Figure 78 exhibit spindles directed longitudinally in the 

 mesenchyme cells A^-^"^, B^'^^ (cf. Figs. 71 and 77), and show that division 

 has been completed in cP-'^'-, c'-^'^ (cf Fig. 75). 



Sections through two other embryos, a little more advanced still in 

 development, show in the muscle cells C'^, D''-^ (cf. Figs. 71 and 72) 

 spindles directed forwards, inwards, and downwards, i. e. about toward 

 the centre of the gastral cavity, a condition which is realized in tlie same 

 cells at a corresponding stage in Clavelina. (See Van Beneden et 

 Julin's ('8G) Figs. 1 a and 1 b.) These facts will aid us in interpreting 

 sections of later stages. 



Figure 80 (Plate XL) exhibits a dorsal view of a stage more advanced 

 than any yet examined. The blastopore has greatly contracted (cf. 

 Fig. 72) and now lies in the posterior half of the embryo. As gastru- 

 lation has progressed, there has taken place an ingrowth of cells round 

 the margin of the blastopore into the inner layer of the embryo. We 

 have already seen how by this process the anterior chorda cells attain a 



