66 PROCEEDINGS OF THE AMERICAN ACADEMY 



of distinction between "archiblast" and "parablast," based on the 

 supposed " free origin " of the nuclei of the latter. He distinguishes 

 two parts in the parablast, a " subgerminal plate " and a peripheral 

 portion (" Rindenprotoplasma ''). 



We have found nothing to justify the original parablast theory of 

 His, nor can we accept the term " parablast " as defined by Kupffer, 

 Klein, Hoffmann, or Waldeyer. The facts presented in this paper 

 justify the opinion that the periblast represents a part of the entoderm. 

 At the outset it is continuous with the bhistodisc, into the margin of 

 which it is progressively concentrated and thus brought under tlie 

 direct action of cleavage. From the 16-cell stage onward it becomes 

 more and more sharply differentiated, until at the conclusion of 

 cleavage it takes the form of a wreath of flattened cells, destined to 

 remain henceforth an independent layer. The nuclei of the cells 

 multiply rapidly by so-called indirect division, and with each division 

 the cells flatten, while their boundaries become less and less distinct. 

 At length a thin nucleated " plasmodium," without any traces of cell 

 limits around the nuclei, is formed. It is a veritable embryonic ento- 

 derm, tlie function of which begins and ends with the absorption of 

 the yolk material. At least, we have thus far failed, as did Hoffmann 

 and Ryder, to find any evidence that this layer shares in forming any 

 portion of the permanent entoderm. From the time this layer be- 

 comes fully differentiated, it remains, at every stage, so perfectly 

 distinct from every other portion of the embryo, that we see no ground 

 for suspecting that it enters into any of the permanent embryonic 

 layers. 



The periblast is then a tme yolk hypoblast, and is therefore, 

 as Ryder hypothetically suggested, in all essential particulars, the 

 homologue of the hypoblast of the yolk-sac of the chick. The chief 

 difference between the bird and the teleost in this respect is, that in 

 the former the periblast is continuous with the permanent entoderro,- 

 ■while in the latter its continuity is broken at a comparatively early 

 date. 



The Origin of the Entoderm. — On the question of the origin of 

 the permanent entoderm of the teleost, the different views admit of 

 being grouped into two great classes, according as they aflirm or deny 

 the participation of the periblast. Each of these classes may be 

 subdivided into two : the first, according as the icholc or a part of 

 the entoderm is derived from the periblast; the second, according 

 as the entoderm is said to arise by delamination, or by invagination, 

 of the margin of the blastodisc. 



