222 W. L. SCLATEE. 



Fig. 9 B represents a section of the same vesicle behind the 

 embryo proper. Here a complete ring of nucleated protoplasm 

 is seen surrounding the space where farther forward would be 

 found the embryo. This is the highest development of the so- 

 called amnion. 



The amnion springs from the basal supporting cells of the 

 embryo, as is asserted by Kennel. 



This growth, which I have called an amnion, following 

 Kennel's nomenclature, does not seem to me to fulfil the con- 

 ditions of an amnion at all. An amnion may be described as 

 a double fold of the non-embryonic area of a blastoderm ( = 

 vesicle wall), which is caused by the sinking of the heavy 

 embryo into the cavity (= yolk-sac or vesicle) filled with fluid, 

 the double folds finally fusing at the top. 



This definition is true both for the amnion of the vertebrate 

 and of the insect. In the case of Peripatus the outgrowth is 

 not a double fold, but a single and thin lamina of protoplasm; it 

 cannot possibly be explained by the mechanical descent of the 

 embryo into the vesicle, since in that case the amnion would 

 be formed on the other side of the embryo from folds in the 

 vesicle wall. 



It seems, therefore, that this so-called amnion of Peripatus 

 has no sort of homology or analogy to the true amnion of 

 insects and vertebrates. As to the use of this structure in 

 Peripatus, it is at present impossible to dogmatize, but it 

 seems to me that, like other embryonic organs, it has some 

 part in the conveyance of nourishment to the embryo from the 

 vesicle and uterus. 



Another embryo — measuring the embryo '12 mm. the vesicle 

 •25 mm. respectively — of about the same size and age as the 

 one above described, is represented in fig, 10 ; it is remarkable 

 for the thinness of the vesicle wall, which consists of a quite 

 slender string of protoplasm with very few nuclei. This and 

 several other examples which I have met with, of the same sort 

 seem to show that the vesicle wall varies much in thickness at 

 different times. 



During the next stage the primary layers begin to form, and 



