CRANGON VULGARIS. 115 



account that would indicate any migration toward the sur- 

 face like that in Crangou. 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 Crangou. According 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, the 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 nucl^is, derived 

 from that originally in the centre of the^^^^^id in this 

 way the blastoderm is formed. It is toj/e rioticed'that 

 the emergence of the internal cells ta'j'Jy^lace first at 

 that portion of the egg known as the aSimaf^ole, and 

 only later do they appear on the other portions. jL^he 

 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 follows : Although many 

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

 bility to connect the nuclei of segmentation with the 



ESSEX INST. BULLETIN, VOL. XVIII. 15 



