Albert C. Eycleshymer and James Meredith Wilson 143 
the posterior in Fig. 37. The general contour of the embryo shows some 
advance beyond the conditions sbown in Figs. 34 and 35. The head re- 
gion shows a considerable increase in the number of ectoblastic cells, in 
the trunk region but two or three layers are present, while posteriorly 
they again show a marked increase in number. Just beneath the super- 
ficial ectoblast (s. ec.), there is now differentiated a second layer of elon- 
gated cells. These cells, however, possess granules which are similar in 
staining capacity to those of the deep ectoblast and have thus been eon- 
sidered as derivatives from the deeper layer. 
In the mass of cells which makes up the anlage of the future brain, 
there is now observed a slight cavity (br. ¢.) which is the first appear- 
ance of a cavity in the central nervous system. In front of this mass of 
cells is a second thickening which has been designated as the pre-cerebral 
mass (p. cb). 
The notochord (ch.) is now well differentiated, being readily distin- 
guished from the surrounding tissues by the loosely scattered arrange- 
ment of its cells. It extends from the undifferentiated caudal mass of 
cells to the anlage of the future optic vesicles. 
The gut has increased through both forward and lateral extension. In 
its anterior portion its dorsal and ventral walls are closely apposed, yet 
they can be readily traced as distinct layers to a point somewhat beyond 
the anterior end of the brain. In its middle and posterior portions it is 
widely open. Just anterior to the line of large yolk cells (b/.) which 
represent the closed blastopore, there is a dorsal diverticulum of the gut 
which has been regarded by others, as well as ourselves, as the homologue 
of Kupffer’s vesicle. 
Embryo Seventy-five Hours After Fertilization. Embryo Covers 
About 150°.—In Figs. 11 and 12 are represented the anterior and pos- 
terior portions of an embryo of this age. The anterior trunk region is 
narrower and two or three protovertebre are now present. Lateral 
thickenings at the anterior end represent the beginnings of the optic 
vesicles. On either side of the anterior end, there is a darkened area 
which represents the lateral extension of the mesoblast. 
An oblique section through the optic thickenings (op. ¢.) is shown in 
Fig. 38. The superficial ectoblast which is now double layered passes 
over these thickenings unmodified. No lumen is present in the central 
nervous system at this level, but in sections intermediate between those 
represented in Figs. 38 and 39 there is a slight fissure present. The 
mesoblast shows as two wide lateral bands (mes.) on either side of the 
neural rod or keel. The foregut is present, but the close approximation 
of its walls makes its lumen obscure. 
