220 LILIAN SHELDON. 



is now for the first time clearly differentiated, and consists of 

 a layer of nuclei surrounded by a very loosely reticulate layer 

 of protoplasm, around the periphery of the inner yolk mass, and 

 just within the ectoderm, except in the region of the somites, 

 where it is subjacent to the splanchnic wall of the latter (figs. 

 19 a — d, End.). The small round bodies are still present, but 

 in much smaller quantities than hitherto, both outside and 

 among the cells of the embryo, from which fact it may be 

 inferred that most of them have been absorbed ; this reduction 

 is shown in all the four figures (19 a — d). 



In the next stage the straightening of the embryo within 

 the shell had considerably progressed, the posterior end only 

 being bent at an angle to the main part of the body. There is 

 as yet no anus, so that the stomodaeum is formed considerably 

 earlier than the proctodeum. The somites are very distinct, 

 with a thin splanchnic (fig. 20, sp.) and a thick somatic wall 

 (fig. 20, so ) ; they do not contain in their cavities any of the 

 small round elements found in them in previous stages. The 

 ectoderm of the lateral body wall, i. e. that covering the 

 somites, is thickened, and in the ventral regions of the thicken- 

 ing rounded elements are present, which will give rise to the 

 future nerve-cords (fig. 20, n. s.). In the central yolk of this 

 embryo (fig. 20) there are traces of the yolk segmentation, some 

 of the segments containing nuclei ; whether these traces have 

 really been retained in this particular embryo longer than usual, 

 or whether in the last few eggs which have been described they 

 have been destroyed by the action of reagents is doubtful ; the 

 latter interpretation, however, seems possible, since in young 

 segmenting embryos whose yolk could in the fresh state 

 be very clearly seen to be segmented, the segmentation was 

 only partially discernible in sections of them when pre- 

 served. 



In all the eggs of stages subsequent to the segmentation 

 which have been described hitherto, it was not possible to make 

 out anything in a surface view of the embryo either in the 

 fresh state or after preservation, owing to the peripheral yolk 

 which surrounded it and completely obscured its external 



