100 J. r. HILT. 



" cliez tons les Cliordes les premiers blastomeres qui se 

 differencient et qui avoisineut le pole animal de I'cBuf sont 

 des elements epiblastiqnes. C'est par la couolie cellulaire qui 

 resulte de la seginentation ulterieure de ces premiers blasto- 

 meres epiblastiqnes que se fait, chez les Sauropsides, Tenve- 

 loppement du vitellus. Dans I'oeuf reduit a n'etre plus 

 qu'une sphere microscopique, Tepibolie a pu s'achever des la 

 fin de la segmentation, voire meme avant Tacbevement de ce 

 pbenomene." The " amas cellulaire interne" (embryonal 

 knot, inner cell mass), Yan Beneden shows, differentiates 

 secondarily into "un lecithophore et un bouton embryon- 

 naire/' The former is the entoderm of other authors, the 

 latter the formative or embryonal ectoderm. Hubrecht, in 

 the forms studied by him (Sorex, Tupaia, Tarsius^) finds 

 a corresponding differentiation. In Tupaia he describes the 

 morula stage as consisting of a single central lightly staining 

 cell, which he regards as the parent cell of the inner cell-mass 

 of later stages, and of a more darkly staining peripheral layer 

 which forms the unilaminar wall of the blastocyst. Here, 

 then, the parent cells of the two cell-groups would appear to 

 be separated at the first cleavage. Hubrecht, like Van 

 Beneden, holds that the inner cell-mass furnishes the 

 embryonal ectoderm and the entire entoderm of the blastocyst. 

 The peripheral layer he has termed the trophoblast ('88, p. 

 511), and in his paper on the placentation of the hedgehog 

 ('89, p. 298) he defines the term as follows : " I propose to 

 confer this name to the epiblast of the blastocyst as far as it 

 has a direct nutritive significance, as indicated by proliferating 

 processes, by immediate contact with maternal tissue, matei-nal 

 blood, or secreted material. The epiblast of the germinal 

 area — the formative epiblast — and that which will take part 

 in the formation of the inner lining of the amnion cavity is, 

 ipso facto, excluded from the definition." Thus the name 



' lu Erinaceus the entodei'in, from Hubreclit's observutions, appears 

 to he precociously ditf erentiated, prior to the separation of the embryonal 

 ectoderm from the overlying trophoblast, but the details of the early 

 developmeut in this form are as yet only incompletely known. 



