ON THE DEVELOPMENT OP SAf4ITTA. 359 



IV. Further Study of the Embryonic Development by 

 MEANS OP Sections. 



In the earliest stages of the embryonic development, 

 sections show very little which cannot be seen in the living 

 embryo. The earliest sections made were of the egg shortly 

 after gastrulation had 1)egun, when the archenteric cavity is 

 still small. They show comparatively few large cells, with 

 moderately distinct cell boundaries, and the nuclei arranged 

 close to the outer limits of the cells, in the epiblast, and at 

 the free surface in the cells lining the archenteron. I have 

 not been able to make out with certainty whether the genital 

 cells are already differentiated at this stage, but in one 

 section two nuclei larger than the rest lie side by side close 

 to the archenteric cavity, and it seems probable that these 

 are the genital cells. In sections of a later gastrula there is 

 no very important change ; the cells are much more numerous, 

 so that there is an almost continuous band of nuclei round 

 the outside of the embryo and round the archenteron. Two 

 of the latter may be seen to be larger than the rest, and 

 project a little into the cavity. At a stage when the 

 archenteron is divided into three branches by the folds, 

 already four genital nuclei are seen, although the cells still 

 appear as two when seen alive ; but as the four are packed 

 so closely together, they would not be easily distinguishable 

 in the living state. (PI. 19, fig. 7, only one genital cell 

 [gen. c] appears in this section.) 



At a period slightly later than this the closing of the 

 blastopore may be seen ; it lies now not quite terminally, but 

 slightly in front of the posterior end, but I do not know with 

 certainty whether dorsally or ventrally, owing to the absence 

 of anything to distinguish the dorsal from the ventral surface 

 at this stage. Observations on the living embryo, however, 

 lead me to believe that it is the ventral surface on which the 

 blastopore comes to lie before it closes. 



As the embryo elongates, and the different layers come 



