436 TUNICATA. 



We have seen that the placenta is formed from the lower parts of 

 the primary hrood-sac, the upper halves of the outer and inner 

 lamellae taking no part in it (Figs. 217 and 218). We are still quite 

 in the dark as to the future fate of these latter parts, which cover 

 the embryo like a cap. According to Barrois (No. 87) and Todaro 

 (No. 110), they are cast off and disintegrate. 



According to Salensky, on the contrary, they are retained by the embryo, 

 becoming closely connected with it and participating in its formation. That 

 part of the outer lamella (Fig. 217, e!) of the primary brood-sac, derived from 

 the maternal atrial wall, which is not concerned in the formation of the 

 placental membrane, is said to yield the ectoderm of the embryo (Fig. 218, ec),* 

 and the upper half of the outer lamella, which consists of flat cells, has there- 

 fore been called by Salensky the ectoderm-germ. The inner lamella (Fig. 217, 

 fw), on the other hand, which can be traced back to the transformed epi- 

 thelium of the oviduct, is said to yield, together with those parts that are not 

 used up for forming the roof of the placenta, chiefly the mesoderm-tissue of 

 the embryo (Fig. 218) ; the enteric rudiments, however, are also said to 

 originate in this layer. We must admit that we feel sceptical as to these state- 

 ments. According to them, the embryo results from separate rudiments derived 

 from various parts of the body of the parent. The epithelium of the atrial cavity 

 of the latter would yield the ectoderm, the oviduct, a part of the mesoderm 

 and the enteric rudiments, while the rest of the embryo would be derived 

 from immigrated follicle-cells (for, according to Salensky, the blastomeres 

 take no part in the building up of the embryo, p. 421). We are inclined to 

 think that errors of observation or of interpretation must here have crept in. 

 [See editorial notes, pp. 420-425]. 



At the stage when we should expect the development of the germ- 

 layers, + and the first rudiments of the organs, there is a considerable 

 gap in our knowledge. We, at least, have been unable, from the very 

 fragmentary statements of the stages that follow those described 

 above, to obtain any clear idea of these processes of development 

 which we desire to compare with the facts known in connection 

 with the ontogeny of the other Tunicates or that of other animals. 



* [This layer, (e') the epithelial capsule of Brooks, is apparently only a 

 temporary protective membrane which disintegrates at a later stage. It takes 

 no part in the formation of the ectoderm, which is, on the contrary, derived 

 from the blastomeres (Brooks, Heider and Korotnefp). — Ed.] 



t In connection with the formation of the germ-layers in the Salpidae, 

 special stress is to be laid on a " gastrula-stage " observed by Barrois (No. 87), 

 in which an invagination is found on the lower side of the embryo, that turned 

 towards the placenta. 



[According to Brooks, the stage in Salpa corresponding to the gastrula of 

 the Ascidians is to be sought in a stage like that given in Fig. 217, the cavity 

 of the follicle, which becomes the body-cavity, being the cleavage-cavity, 

 and the blastopore coinciding with the attachment of the central mass of 

 blastomeres and follicle-cells to the inner layer of the brood-chamber. The 

 segmentation of the egg is much retarded, but the gastrula is planned out 

 in follicle-cells. This view is not accepted by other observers. — Ed.] 



