392 



On the Embryo of Peripatus Capensis. [Dec. 14, 



embryos have been drawn by Miss Balfour, and will be figured in the 

 first part of the forthcoming memoir above referred to. 



Origin of the Mesoblast. — Balfour's three remaining drawings are, 

 as already stated, representations of transverse sections of the embryo 

 figured by him as a whole. They show that, as he stated in the 

 letter quoted, the mesoblast originates as a series of paired outgrowths 

 from the hypoblast, and that these outgrowths are formed near the 

 junction of the hypoblast with the epiblast at the lips of the blasto- 

 pore. The mesoblast can be seen in the actual sections to have the 

 form of paired sacs, the cells forming which pass continuously unto 

 those of the hypoblast. One of them distinctly shows that at the 

 stage with six somites, communications exist between the cavity of 

 the mesenteron and that of the mesoblastic somites, and there is no 

 need for us to enlarge upon the importance of these facts. Their 

 close bearing upon some of the most important problems of morphology 

 will be apparent to all, and we may, with advantage, quote here some 

 passages from Balfour's " Comparative Embryology," which show 

 that he himself long ago had anticipated and in a sense predicted 

 their discovery. 



" Although the mesoblastic groove of insects is not a gastrula, it is 

 quite possible that it is the rudiment of a blastopore, the gastrula 

 corresponding to which has now vanished from development." " Com- 

 parative Embryology," vol. 1, p. "678. 



" Tkacheata. — Insecta. It (the mesoblast) grows inwards from the 

 lips of the germinal groove, which probably represents the remains of 

 a blastopore." " Comparative Embryology," vol. 2, p. 291. 



" It is, therefore, highly probable that the paired ingrowths of the 

 mesoblast from the lips of the blastopore may have been, in the firs t 

 instance, derived from a pair of archenteric diverticula." " Compara- 

 tive Embryology," vol. 2, p. 294. 



They were discovered in June last, only a short time before he 

 started for Switzerland ; we know but little of the new ideas which 

 they called up in his mind. ' We can only point to passages in his 

 published works which seem to indicate the direction which his specu- 

 lations would have taken. 



" In the first place it is to be noted that the above speculations render 

 it probable that the type of nervous system from which that found in 

 the adults of the Echinodermata, Platyelminthes, Chsetopoda, Mol- 

 lusca, &c, is derived, was a circumoral ring, like that of Medusae, 

 with which radially- arranged sense-organs may have been connected ; 

 . . . . Its anterior part may have given rise to supra- oesophageal 

 ganglia and organs of vision ; these being developed on the assump- 

 tion of a bilaterally symmetrical form, and the consequent necessity 

 arising for the sense-organs to be situated at the anterior end of the 

 body. If this view is correct, the question presents itself as to how 



