CRANGON VULGARIS. 13 



which takes place in Crangon at an earlier date than in As- 

 tacus. The muscular system of the "stomach," on the other 

 hand, is much later in development than in the form stud- 

 ied by Reichenbach, as the walls do not before hatching ac- 

 quire a marked muscular appearance. 



ENTODERM. From the time of gastrulation until short- 

 ly before stage H, the history of the invaginated entoderni 

 can be briefly told. At first the cells remain in proximity 

 to the blastopore or anus, but (fig. 29) they have no inti- 

 mate connection with the rest of the germ. They are, 

 rather, isolated cells in the midst of a large mass of yolk, 

 each cell consisting of a comparatively large nucleus sur- 

 rounded by a thin pellicle of protoplasm which exhibits a 

 tendency to extend in pseudopodal prolongations at the 

 angles. 



With development the entoderm cells wander farther 

 from the point of origin and remain scattered through the 

 yolk, for a longtime uniting neither with their fellows nor 

 with the other germ layers. Their division is not rapid 

 until stage G is reached, when they begin to multiply more 

 rapidly and to give rise to an epithelium by joining them- 

 selves together. 



Owing to the solidity of the yolk and the absence of 

 well defined yolk balls in Crangon, it is difficult to ascer- 

 tain the relationships of the entoderm cells to the yolk in 

 their pre-epithelial stages. Several facts, however, lead 

 me to the view that they are not to be regarded as the 

 centres of yolk balls, but rather as forming a potential if 

 not an actual reticulum, in the meshes of which the yolk 

 balls occur. This view is in full accord with that of May- 

 er ('77, p. 237) of the relations of the entoderm cells in 

 Eupagurus and in Porcellio (Reinhard, '87) ; but differs 

 from Palsemon where Bobretzky ('73) found these cells 

 forming the centres of yolk balls. 



