CRANGON VULGARIS. 13 



which takes place in Cnuigon at an cailier date than in As- 

 tacns. 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- 

 qnire a marked muscular appearance. 



Entoderm. — From the time of gastrulation until short- 

 ly before stage H^ the history of the invaginated entoderm 

 can be briefly told. At lirst 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 long time uniting neither with their fellovvs nor 

 with the other germ layers. Their division is not rapid 

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

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

 selves together. 



Owing to the solid ity of the yolk and the absence of 

 well delined 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 

 nie 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 füll 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 PaUemon where Bobretzky ('73) found these cells 

 forming the centres of yolk balls. 



