30 SALPA. 



On the formation of the body cavity of the embryo a central axial 

 portion of the placenta becomes separated from a peripheral layer ; and a 

 channel is left between them which leads from a maternal blood sinus into 

 the embryonic body cavity. The peripheral layer of the placenta is formed 

 of cells continuous with the epiblast of the embryo ; while the axial portion 

 is constituted of a disc of cells adjoining the embryo, with a column of 

 fibres attached to the maternal side. The fibres of this column are believed 

 by Salensky to be products of the original rudiment of the placenta. The 

 placenta now assumes a more spherical form, and its cavity becomes shut off 

 from the embryonic body cavity. The fibrous column breaks up into a 

 number of strands perforating the lumen of the organ, and the cells of the 

 wall become stalked bodies projecting into the lumen. 



When the larva is nearly ready to become free the placenta atrophies. 



The placenta functions in the nutrition of the embryo in the following 

 way. It projects from its first formation into a maternal blood sinus, and, 

 on the appearance of a cavity in it continuous with the body cavity of the 

 embryo, the blood of the mother fully intermingles with that of the embryo. 

 At a later period the communication with the body cavity of the embryo is 

 shut off, but the cavity of the placenta is supplied with a continuous stream 

 of maternal blood, which is only separated from the foetal blood by a thin 

 partition. 



It is now necessary to turn to the embryonic development about which it 

 is unfortunately not as yet possible to give a completely satisfactory account. 

 The statements of the different investigators contradict each other on most 

 fundamental points. I have followed in the main Salensky (No. 34), but 

 have also called attention to some points where his observations diverge 

 most from those of other writers, or where they seem unsatisfactory. 



The development commences at about the period when the brood-pouch 

 is becoming formed ; and the ovum passes entirely into the brood-pouch 

 before the segmentation is completed. The segmentation is regular, and 

 the existence of a segmentation cavity is denied by Salensky, though 

 affirmed by Kowalevsky and Todaro 1 . 



At a certain stage in the segmentation the cells of the ovum become 

 divided into two layers, an epiblast investing the whole of the ovum with the 

 exception of a small area adjoining the placenta, where the inner layer or 

 hypoblast, which forms the main mass of the ovum, projects at the surface. 

 The epiblast soon covers the whole of the hypoblast, so that there would 

 seem (according to Salensky's observations) to be a kind of epibolic 

 invagination : a conclusion supported by Todaro's figures. 



At a later stage, on one side of the free apex of the embryo, a 

 mesoblastic layer makes its appearance between the epiblast and hypoblast. 

 This layer is derived by Salensky, as it appears to me on insufficient 

 grounds, from the epiblast. Nearly at the same time there arises not far 



1 From Todaro's latest paper (No. 39) it would seem the segmentation cavity has 

 very peculiar relations. 



