CRANGON VULGARIS. 141 



reader may have laiidmarks to guide hitn in the discussion 

 of the internal development. 



Imniediately after the gastrulation, the embryo begins 

 to be outlined. As will be seen by fig. 8, the germi- 

 nal area is mostly in front of the blastopore and is char- 

 acterized by smaller and more closely placed cells. It is 

 here that the most marked changes first take place. The 

 next stage is represented in fig. 10 ; a larger and more 

 detailed representation of this stage is shown in fig. 1 of 

 my other paper ('87). At the posterior end of the egg 

 is shown a broad, somewhat kidney-shaped diso, ta, the 

 representative of the gerrainal area in fig. 8, and which, 

 following Keichenbach, I call the thoracico-abdominal 

 area. The blastopore closed in the median line of the 

 posterior portion of this disc. From the two anterior 

 angles of this disc, two cords of smaller cells extend out- 

 wards and forwards, each terminating in an ovjil disc or 

 plate of still smaller cells (ol), the rudiments of the optic 

 lobes for whose subsequent history the reader is referred 

 to the paper just quoted. In the fresh egg treated with 

 dilnte alcohol, this somewhat U-shaped germ is brought 

 into strong relief, while staining shows that it is difl:eren- 

 tiated from the rest of the blastoderm by the smaller size 

 of the cells and the consequently closer position of the 

 uuclei. The cells are smaller in the optic lobes and in the 

 thoracico-abdominal area tlum in the cords connecting 

 them. In section these cells are all more columnar thau 

 those of the uudifferentiated blastoderm, wliich are very 

 flat and much as in the earlier stages. 

 j, This stage of the egg, which I may call A, corresponds 

 rather closely with ßeichenbach's ('86) stages A — D, 

 with the following exceptions : Most noticeable is the 

 fact that I have not seen the optic lobes in Crangon be- 

 fore the closure of the blastopore, while Keichenbach (J.c,, 



