XVII PKOTOCHOEDATA 635 



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 and pericardium of the embryo, whilst the rest of the vesicle 

 disappears. 



On the floor of the pharynx the endostyle is formed as a large 

 median groove with thick prominent lips, and behind this there 

 grows out what Julin calls a prolongation of the pharynx, but which 

 we (following Hjort) regard as two enormous epicardial tubes fused 

 together. This prolongation curls itself round the yolk, and from it, 

 in a manner to be described shortly, the first buds arise. It remains 

 to be added that, when the atrial involutions have become united with 

 the median cloacal opening, the two rudimentary gill-shts disappear 

 and are replaced by a single dorsal median opening into the pharynx. 



If this description has been followed, it will be seen that the first 

 individual to be formed by the development of the fertilized egg in 

 Pyrosoma is of a very degenerate character. It possesses no trace of 

 a mouth or of an intestine, and no trace of notochord or caudal 

 muscles. In the neighbourhood of the blastoporal rim a ring of 

 mesenchyme is produced. This vestigial individual is called the 

 Cyathozooid, in order to distinguish it from the more perfect 

 individuals produced by budding. 



When we pass to the Thahaceae we find, in the genus Salpa, a 

 form in which the egg develops directly into the adult form without the 

 intervention of a tailed larval stage. In this case the egg remains 

 in the oviduct in the mother, only moving down from the ovary to 

 a brood-chamber at the mouth of the oviduct ; whereas in Pyrosoma 

 the egg is spawned into the sea. The egg, after dividing into a few 

 blastomeres, is invaded by follicle cells which wander in between the 

 blastomeres and surround them. According to Salensky (1883), the 

 blastomeres perish and the embryo is built up out of follicle cells. 



In view of the fact that Salensky (1891) has made somewhat 

 similar statements about Pyrosoma, in which he alleges that test cells 

 become blastomeres, and that Juhn has shown this view to be 

 erroneous, it may be doubted whether Salensky's statements about 

 Salpa are any more reliable. Moreover, in the case of Salpa, it is very 

 difficult to distinguish between blastomeres and follicle cells. It 

 seems more probable that it is the follicle cells which perish and are 

 eaten by the separated blastomeres, which then reunite. Similar 

 cases of the separation and subsequent reunion of blastomeres are 

 known amongst Platyhelminthes. 



The solid mass of cells constituting the embryo sits, rider-like, on 

 a knob of maternal tissue, inside which there is a great development 

 of blood-vessels. This knob constitutes a nutritive organ or placenta. 

 The blood of the mother seems to diffuse through to the tissues of 

 the embryo. An ectoderm is differentiated from the peripheral 

 layer of the embryonic mass, and the placental knob is overgrown by 

 a flap of this layer. From a thickening of the ectoderm the nervous 

 system is developed. 



Two lateral portions of the inner or endodermic mass become 

 distinguishable from the rest, by their histological character, iis 



