ON THE EMBRYOLOGY OF THE TELEOSTS. 199 



Soon after this number was reached, lower layer cells were noticed. We did not con- 

 clusively settle the manner in which they arose but are inclined to believe that the greater 

 portion arose from the already formed cell elements of the blastoderm while possibly a 

 small proportion had their origin in the free nuclei of the yolk. With their formation the 

 epiblast becomes differentiated and at first consists of a single layer of cells. These cells 

 in vertical optical section are lens shaped while the lower layer cells are polygonal in out- 

 line on account of their mutual pressure. 



At first the blastoderm fits as a cap over the yolk but soon by the proliferation of cells 

 it acquires a lenticular shape and is seated in a concavity in the surface of the deuto- 

 plasmic portion of the egg. At this time the intermediary layer (Parablast of Klein 

 not of His) is plainly seen and its thickened margins (bourrelet peripherique of Van 

 Bambeke) is very conspicuous. m Regarding the origin of this layer we have nothing new 

 to offer. Its first appearance was not noticed either as to the exact stage or as to the 

 method in which it arose. 1 Neither was the time of its disappearance observed ; it was 

 visible until the blastoderm nearly covered the yolk. When first seen the layer was clear 

 and transparent without any traces of granules, vacuoles, nuclei or cells, though at a later 

 stage they were visible. The first observer who noticed this intermediary layer was Lere- 

 boullet who saw it both in the pike ('54 p. 248) and perch (1. c, p. 250). He describes 

 it in the latter as follows : " II existe sous le blastoderme une membrane particuliere, dis- 

 tincte, composee de grandes cellules tres pales ; c'est d'elle que se formeront les organes 

 abdominaux";.and in speaking of the pike he says that the vitelline globules are changed 

 to this layer. Almost all subsequent observers have seen this same layer and have added 

 to our knowledge of it. Our discussion of their results will be taken up in connection 

 with that of the germ layers with which it is intimately connected. 



At about the time of the differentiation of the lower layer cells as well as at later stages 

 free nuclei were seen on the surface of the yolk. These nuclei were irregularly arranged, 

 in fact no traces of any regularity could be discerned except that all were on the surface 

 and now were to be seen on the interior of the yolk. (The term surface here embraces 

 not only that portion which is in contact with the egg membranes but also that on which 

 the intermediary layer rests). In the eggs of Merlucius at a slightly older stage similar 

 nuclei were seen and around many of them, especially those nearest the blastoderm, the 

 cell walls could be made out, the whole presenting an appearance somewhat similar to 

 that given by Kupffer ('68 p. 217 pi. xvi) in the eggs of Gasterosteus. These free nuclei 

 and cells are not arranged with anything like the regularity of Kupffer' s figure's. In the 

 dinner egg I watched the process of cell formation around these nuclei with some 

 care. The nuclei nearest the germinal portion of the egg were the first to become the 

 centres of cells and the formation of the cell boundaries took place in a corresponding 

 direction, that is those portions of each cell wall nearest the blastoderm appeared first and 

 these gradually extended themselves around the nuclei. The whole operation required 

 over half an hour. In the dinner but comparatively few of these free nuclei and 

 resulting cells were seen. 2 



*a 



'The observations of 1882 elsewhere detailed alter this representing a portion of the blastoderm and the adjacent 



statement slightly. portion of the peripheral cushion; in the latter there being 



2 In 1882 by staining, these free nuclei with all their accom- shown asters, amphiasters, and the process of outlining of the 



panying phenomena were seen and studied, fig. 24*, pi. xv, cells. 



