UROCHORDA. 25 



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 obser- 

 vations diverge most from those of other writers, or where they seem 

 unsa tisfactory. 



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 ineso- 

 blastic 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 

 from the same point of the embryo, but on the opposite side, a solid 

 thickening of epiblast which forms the rudiment of the nervous system. 

 The nervous system is placed close to the front end of the body; and nearly 

 at the opposite pole, and therefore at the hind end, there appears immedi- 

 ately below the epiblast a mass of cells forming a provisional organ known 

 as the elaeoblast. Todaro regards this organ as mesoblastic in origin, and 

 Salensky as hypoblastic. The organ, is situated in the position which 

 would be occupied by the larval tail were it developed. It may probably 

 be regarded (Salensky) as a disappearing rudiment of the tail, and be 



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

 very peculiar relations. 



