1886.] Embryology. 181 
otic sack is never formed, because, in the first place, the intra-oval 
period of development does not last long enough; nor,in the second 
place, is it possible, owing to the comparative small size of the 
yolk, and the rapid growth of the embryo, for the latter to 
become bodily invaginated into the blastodermic vesicle, which is 
filled with yolk. The amniotic folds can, therefore, not meet 
upon the middle line of the back, and coalesce, as they do in the 
higher endocyemate forms. The development of a transient 
amniotic head-fold of greater width and in advance of the side 
and tail folds, is also prevented by the absence of a strongly 
marked cranial flexure in the embryos of Teleosts. 
The mechanical effect of the gradual development of the cranial 
flexure in exaggerating the development of the amniotic head- 
fold in the Chordata, will be best appreciated by a glance at 
diagrams 1, 11, 111, and Iv, representing respectively the brain of an 
acraniate, a marsipobranch, an elasmo- 
branch anda mammal. With the increase ©——————= I 
in the volume and area of the cerebral 
cortex, which occurs mainly on the dorsal i arate m 
and lateral aspects of the anterior end of 
the neurula, the acceleration of growth of Ir 
the brain substance also occurs on those JE 
aspects, and a downward flexure of the 
floor of the brain necessarily takes place. zë 
The rapid enlargement of the cephalic 
end of the embryo of an endocyemate, 
eutherian or paratherian form, and the rapid 
or precocious development of the cranial 
flexure, would naturally, in such a type, tend to cause the amni- 
otic head-fold to be developed earlier and to a greater extent 
than the tail-fold, as is shown in Fig. B, at a. 
In the eutherian types, with inverted germinal layers, an amni- 
otic head-fold of the kind developed in normal forms is never 
rmed, because the cavity of the true amnion in the former is 
developed by the vacuolization or the formation of a cavity or 
cavities in the solid epiblastic mass, and not by invagination. In 
the Tracheates possessing an amnion there is no cephalic flexure, 
and the part of the amnion which is first developed in the most 
Pronounced manner is often the tail-fold, due apparently to the 
ingrowth of the caudal end of the embryo into an involution of 
ê blastoderm, confined in a rigid egg-envelope, the involution 
being thrust into the yolk. Later, with the growth and encroach- 
ment of the head-end of the embryo upon the yolk, the abdomen is 
again everted in some cases from its amniotic sack. In Peripatus 
edwardsii, according to Von Kennel, cleavage is total, the devel- 
PAON is viviparous and intra-uterine, a hollow blastula is 
ormed, the embryonic area at one pole of the blastula is invagi- 
nated into the latter, so that the ventral surface of the embryo is 
VOL, XX.—NO. IL “7 
