198 Transactions of the 



that the elements of the subgerminal processes separate themselves 

 from the blastoderm, and remain on the bottom of the cavity the 

 more the blastoderm is lifted off; and thirdly, that the further the 

 eo"g is developed, the more these elements lying on the bottom of 

 the cavity come outwards, so as to increase the peripheral thickening 

 of the blastoderm (see Figs. 11 and 12). 



The more the blastoderm grows round the yolk globe, the more the 

 former becomes thinner in its central part. If we look at a section 

 through the blastoderm of fourteen days, we observe that the central 

 portion of the blastoderm is composed only of two layers of cells ; the 

 superficial layer consists of flattened cells, the deeper layer of spherical 

 cells arranged more or less loosely in a row. The former layer is 

 continued into the similar one of the peripheral thickening, the latter 

 into the second stratum of the peripheral thickening ; here this 

 second stratum contains one or two layers of more or less cylindrical 

 cells. Below these two strata the peripheral thickening contains a 

 few more layers, at least two of spherical, distinctly granular, larger 

 cells, with one or two nuclei. The cells of these latter layers are 

 continuous with the large elements which lie on the bottom of the 

 cavity (in this stage of development of course only at the peri- 

 pheral part of the cavity; in the central part of the bottom of the 

 cavity there are no more or very rarely such elements to be found). 



About the fifteenth or sixteenth day the blastoderm has grown 

 round somewhat more than a quarter of the yolk. In fresh eggs as 

 well as in those hardened in chromic acid the contrast between the 

 thickened peripheral part and the rest of the blastoderm is very 

 striking. In eggs hardened in chromic acid, and deprived of their 

 vitelline membrane, the former appears as a whitish ring ; the rest of 

 the blastoderm is so thin that the fat-globules of the yolk shine 

 through it. About this time we observe that at one spot of the peri- 

 pheral thickening a knob-shaped prominence shows itself which slopes 

 towards the centre. This prominence represents the first trace of 

 the embryo. In the subsequent stages (one day after) the embryonal 

 prominence has already increased so far that it stretches like a cord 

 from the elevated border towards the centre of the germinal area. 

 We may now also observe with a lens that the end directed towards 

 the centre is rounded, and fm-ther that the embryonal prominence 

 projects beyond the extreme contour of the elevated border of the 

 blastoderm in the form of a small knob. At the point of junction of 

 the cephalic third (i. e. the portion turned towards the centre of the 

 blastoderm) with the rest of the embryonal prominence we observe a 

 shallow groove, the first indication of a dorsal groove, which is con- 

 tinued towards the centre (the head) and towards the periphery (the 

 tail) a short distance, becoming more shallow, so that it may be 



