Primitive Cell-layers of the Embryo. 331 



distinct from the cleavage-process (free-cell formation). A 

 furtlier comprehension of the accompanying conditions and 

 mode of carrying out of the suj)pression of steps in the historical 

 epitome of the individual's development will, more than any 

 thing else, tend to this result. The non-identity of the mouth 

 in Diploblastica and Triploblastica is one of the most curious 

 divergences which a comparison of the two groups brings out. 



There is on the whole a satisfactory concordance of testimony 

 with regard to the chief tissues and organs to which the three 

 layers respectively give rise, if we except the generative 

 products. The hypoblast of the Triploblastica retains the 

 characters and significance of the Diploblast's endoderm. The 

 fundamental properties of the hitter's ectoderm .(musculo-sen- 

 sorial layer of Kleinenberg) become distributed between the 

 tissues differentiated from epiblast and mesoblast — a fact which, 

 whether rightly or wrongly, suggests the ectoderm as the true 

 source of origin of the mesoblast \ and, in the case of the earth- 

 worm, Kowalewsky's researches demonstrate this origin con- 

 clusively. 



That the generative products arise from cells of the ectoderm 

 in Hydra is certain, from Kleinenberg's careful observations. 

 Hackel, on the other hand, has found them derived from the 

 endoderm in certain Medusa and in Calcareous Sponges, whilst 

 Allman makes the same statement as to some Hydroid polyps. 

 That the ovaries and testes in higher animals arise from the 

 outer layer is not inconsistent with the fact that they may first 

 definitely appear within the limits of the mesoblast. An in- 

 growth and intercalation of the cells of the epi- and mesoblast 

 at an early period, such as Waldeyer has pointed to, sufficiently 

 explains the position of the vertebrate ovary and testis, even 

 tliough they be developed from the epiblast. The position of the 

 generative masses of Oligochtetous Annelids in their earliest 

 phase, as buds of the tissue in immediate contact with the 

 nerve-cord, to which I have drawn attention in Chcetogaster* 

 and Tuhifex'\, is in complete agreement with the view of their 

 derivation from cells of the epiblast, when considered in the 

 light of Kowalewsky's admirable demonstration of the ingrowth 

 of the epiblast to form the ganglion-chain of Lumbricus and 

 £uaxes. 



A true blood-system, or blood- lymph-system as it is better to 

 call it in view of the present signification of words, is only 

 possible where a mesoblast is developed — that is, in the Tri- 

 ploblastica. In all Triploblastica it is represented by lacunas 

 or channels, or by mere wide-setting of the cellular elements 



* Quart. Journ. Microsc. Science, July 1870. 

 t Auu. & Majr. Nat. Hist. 1871, vii.'p. 90. 



