CRANGON VULGARIS. 115 



account that would iiidicate any migration toward the sur- 

 face like that in Crangon. The origin of the nuclei of 

 the "yolk cells" was not traced. 



In the Spiders, according to both Ludwig ('76) and Locy 

 ('86), the process of segmentation is readily brought 

 into accord with that in Crano^on. Accordinor to the lat- 

 ter author, the segmentation nucleus, surrounded with a 

 mass of protoplasm which sends off processes among the 

 yolk granules, occupies a position in the centre of the 

 egg ; while the outside of the egg is covered with a thin 

 layer of non-nucleated protoplasm, the blastema, tlie ex- 

 istence of which was denied by Ludwig. At the first 

 segmentation, this nucleus divides into two and with it 

 the protoplasm also divides, while traces of a similar seg- 

 mentation can be seen in the deutoplasm. These nuclei 

 now occupy a subcentral position in the egg. At the 

 eight-cell stage, the nuclei are still nearer the centre than 

 to the surface ; and even when the egg has at least thirty 

 cells, none have emerged at the surface. Later, they do 

 appear and then the contiguous protoplasm of the blas- 

 tema unites with that surrounding the nucleus, derived 

 from that originally in the centre of the egg, and in this 

 way the blastoderm is formed. It is to be noticed that 

 the emergence of the internal cells takes place first at 

 that portion of the egg known as the animal pole, and 

 only later do they appear on the other portions. The 

 bearings of this will appear a little farther on. 



In the case of the Hexapods the great bulk of the evi- 

 dence is certainly in favor of the view of segmentation 

 which I have thus shown to be the case in spiders and dec- 

 apods. Without attempting an exhaustive review, we may 

 summarize our knowledge as foUows : Although many 

 writers (e. g., Korotneff, '85, p. 571) confess their ina- 

 bility to connect the nuclei of segmentation with the 



ESSEX INST. BULLETIN, VOL. XVIII. 15 



