124 DR. E. KLEIN. 



edge of the segmentation cavity. The peripheral thickening 

 of the archiblast rests on the surface of the thickened para- 

 blast, both being in very close contact. Sometimes (see the 

 left side of fig. 4) the boundary between the two is indistinct, 

 and then it appears as if the deepest elements of this part of 

 the archiblast were directly developed out of the parablast, 

 especially if one bdars in mind that numerous nuclei are 

 contained in this portion of the parablast, which (nuclei), 

 as will be more minutely described hereafter, are similar to 

 those of the cells of the archiblast. 



Whether this definite boundary between the two structures 

 (parablast and thickened part of archiblast) is a sufficient 

 reason for saying that the deepest cells of the archiblast of 

 this stage develop out of the parablast — the substance of the 

 latter becoming differentiated round its nuclei into cell-terri- 

 tories — is a matter which I am not in a position to decide 

 definitely ; it is, however, very probably so, inasmuch as in 

 only a little later stage cells may be followed with certainty to 

 develop out of the parablast. This will be stated hereafter. 



From the inspection of fig. 4 it is also clear that the 

 thickened part of the parablast is not situated symmetrically 

 in all parts in relation to the archiblast, e. g. on the right side 

 it is situated more externally than on the left. This thickened 

 portion of the parablast contains very numerous nuclei ; they 

 are more numerous in the superficial than in the deeper parts. 



In a somewhat later stage (thirteen days) the archiblast is 

 seen to be separated, for nearly its whole extent, from the 

 yolk by the segmentation cleft (see fig. 5). The elements of 

 which the archiblast now consists are little smaller than 

 those in the former stage, except the most superficial layer, 

 which is composed of slightly flattened cells : all the other 

 layers are made up of polyhedral cells. At and next to the 

 peripheral thickening of the archiblast, there are present on 

 its lower surface numerous more or less spherical elements 

 loosely connected with each other ; some of them are much 

 larger than the ordinary embryo-cells, and possess two, 

 three, and more nuclei, besides yolk granules. I have seen 

 some of these cells contain as many as six nuclei, each with a 

 nucleolus. These nuclei were arranged like the sections of one 

 large lobed nucleus. But also in other parts, on the lower 

 surface of the archiblast, isolated cells may be found of the 

 same character as those just mentioned (see figs. 5, 6, 7, and 8). 

 Except the difference of the most superficial layer of cells, the 

 archiblast does not present as yet any differentiation of its 

 elements into strata. 



The parablast is seen to have assumed much greater 



