BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
(b) Closure of the Blastopore. 
In Figure 78 (Plate XI.) is shown a section parallel and slightly lat- 
eral to the modian plane of an embryo a littlo 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 auterior (left in the figure) than toward the posterior 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®", 4515). 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 tho nerve cells. They then 
form a plate of eight cells lyiug in the dorsal wall of the archenteron 
anterior to tho blastopore. 
The endoderin cells of this embryo (Plate XI. Fig. 78) have under- 
gone no new divisions since the 128-ccll stage (Figs. 73-77), though 
their nuclei have considerably increased in size, as is invariably the 
case during the resting period. More lateral sections than the one 
shown in Figure 78 exhibit spindles directed longitudinally in the 
mesenchyme cells 48% PB (cf, F 
has been completed in dB, e™? (cf. Fig. 75). 
igs. 71 and 77), and show that division 
Sections through two other embryos, a little more advanced still in 
development, show in the muscle cells 07%, D (ef. Figs. 71 and 72) 
spindles directed forwards, inwards, and downwards, i. e. about toward 
the contre of the gastral cavity, a condition which is realized in the same 
cells at a corresponding stage in Clavelina. (See Van Beneden et 
Julin’s (86) Fis 
sections of later stages. 
la and 10.) These facts will aid us in mterpreting 
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 
