420 PROFESSOR LANKESTER. 



same point (fig. 16). In any case they maintain a complete inde- 

 pendence of the coelomic "parentera/' and only apply them- 

 selves at a later period to these outgrowths, in order to form the 

 musculature of the walls of the canals and cavities to which 

 the archenteron gives rise (fig. 17). 



The condition of things in reference to the origin of the 

 musculo-skeletal cells seen in the Holothurians may be ex- 

 plained as a derivative of the delamination process seen in 

 the formation of the identical tissues of Zoophytes, by the 

 application of the hypothesis of precocious segregation. 

 Just as the delaminate origin of the endoderm gives place 

 to an invaginate origin, owing to the early segregation of the 

 two elements, so the ectodermic cells at this later stage of 

 evolution cease to develop the musculo-skeletal cells by dela- 

 mination, and at a time when the embryo is composed of 

 only twelve, eight, or even two cells, the molecules possessing 

 by heredity the power of giving rise to musculo-skeletal 

 tissues are segregated from the cells destined to form ecto- 

 derm pure and simple, which now, indeed, having lost its 

 muscnlo-skeletal elements, should be distinguished from the 

 primitive ectoderm, as epiblast.^ This segregation taking 

 place before the pseudoblastula is formed (in rare cases of 

 retention of the ancestral delaminate origin of the enteric 

 cells, the musculo-skeletal tissue also originates by delami- 

 nation) we find that the greater portion of the cells forming 

 the wall of the pseudoblastula are purely epiblastic, and never 

 give rise to musculo-skeletal progeny. But at one part of 

 the pseudoblastula are the cells containing the segregated 

 musculo skeletal molecules, and others containing the seg- 

 regated endodermic molecules. In Holothurians the two 

 sets of precociously segregated molecules form distinct cells, 

 and thus at an earlier or a later ])eriod we see the musculo- 

 skeletal tissues of these animals originating from cells in the 

 neighbourhood of that area of invagination by which the 

 cavity of the archenteron is formed. 



Admitting this hypothesis of the precocious segregation of 

 musculo-skeletal molecules to be true, it is clear enough that 



1 As Prof. Allen Thomson has recently pointed out, ectoderm and endo- 

 derm correspond to ei)iblast and iiypoblast, plus the share which ectoderm 

 and endoderm have in the mesoblast (Brit. Ass., Plymouth, President's 

 Address, 1877). In fact, we arrive now by the light of minute investiga- 

 tions of Invertebrate ontogeny at a striking confirmation of tiie views" of 

 von Baer. The primary cells of the embryo differentiate into two layers, 

 the ectoderm and the endoderm, or deron and enteron. Each of these 

 again divides into two : the ectoderm into epiblast and musculo-skeletal 

 tissue, the endoderm into hypoblast and ccclomic cpilhelium (parenteric 

 outgrowths). 



