BLASTOSmERE. 



113 



uieiit reach the periphery, and the fatty and often clarkly-granulai 

 food yolk comes to constitute the central mass of the egg (Insects). 



As various as the forms of segmentation are the methods by which 

 the segments are applied to the building up of the embryo. Fre- 

 quently in cases of equal segmentation the segments arrange them- 

 selves in the form of a one-layered vesicle, the blastosphere, the 

 central cavity of which not rarely contains fluid elements of the food 

 yolk ; or they are at once divided into two layers around a central 

 cavity containing fluid; or they form a solid mass of cells without 



FIG. 107. Six stages in the segmentation of a spider's egg (Philodromus limbatus) after Hub 

 Ludwig. A, egg with two deutoplasmic rosette-like masses (segmentation spheres) ; i', 

 the rosette-like masses with their centrally placed nucleated protoplasm without egg 

 membrane ; C, egc 1 with a great number of rosette-like masses ; Z>, the rosette-like masses 

 have the form of polyhedral deutoplasmic columns, each of which has a cell of the blas- 

 toderm lying immediately superficial to it ; E, stage with blastoderm completely formed : 

 F, optical section through the same. The yolk columns form within the blastoderm a 

 closed investment to the central space. 



any central cavity. In numerous cases, especially when the food 

 yolk is relatively abundant (unequal and partial segmentation) or the 

 food supply continuous, the embryonic development is longer and 

 more complicated. The embryonic rudiment in such cases has at 

 first the form of a disc of cells lying on the yolk ; it soon divides into 

 two layers, and then grows round the yolk. 



