528 ALFRED C. HADDON. 



to be an invagination from the epiblastic cells (" hypodermal 

 layer ") at the apex of the developing bud^ whilst the underlying 

 originally solid mass of cells \vhich have primitively proliferated 

 from the parental stomach and the pedal gland with the other 

 internal structures, are modifications of migrated mesoblast. 

 The development of the oesophagus as a depression from the 

 hood also favours the interpretation, as does Vogt's aceount, 

 if we except his earliest stages. 



Subsequently, Prof. M. Salensky examined the gemmation 

 of Loxosoma crassicauda (9). He describes the first stage 

 as consisting of a small group of cells surrounding one central 

 one. The latter by division forms a central mass which attaches 

 itself to the anterior end of the lengthening and pedunculated 

 outer wall of the bud. A slight longitudinal fissure appears in 

 the ectoderm (epiblast of the bud), which is the rudiment of 

 the orifice of the hood. The central mass becomes hollow and 

 forms the hood and the whole of the digestive tract. " The 

 rudiment of the digestive tube presents itself under the form of 

 a cul-de-sac, in which two parts can be distinguished. . . . The 

 superior part is the rudiment of the intra-tentacular depression, 

 the inferior part is the rudiment of the digestive tube and of 

 the rectum. . . . The superior part appears as a sac open in front. 

 The edges of the aperture by which the sac opens now consists 

 of ectoderm and endoderm which are completely united," p. 21. 

 By "endoderm " Salensky means the inner layer of the double- 

 layered bud, which tissue, according to him, forms the inner 

 epithelium of the alimentary tract, the intra-tentacular space 

 and the inner surfaces of the tentacles, their outer surface being 

 formed at the expense of the ectoderm, the tentacles themselves 

 arising just where these two layers fuse. On p. 19, he says, 

 ''the ectoderm and endoderm have arisen from the ectoderm or 

 the integument of the mother. This fact is so clear to anyone 

 who observes the profile of young buds of Loxosoma, that there 

 cannot exist any doubt as to its reality. From the analogy 

 which exists between all the species of Loxosoma, 1 may affirm 

 that the described phenomena should be common to all the 

 species." 



