CRANGON VULGARIS. 137 



solid invagination. The authors at first regard these in- 

 vaginated cells as entoderm but later as ento-mesoderm. 

 From the low er surface of this layer many cells bud off 

 and sink into the yolk to form yolk cells, which, the au- 

 thors say, play no part in the development of the tissues, 

 but act as solvents of the yolk. Later, the true entoderm 

 is formed by the Separation of a layer of cells from the 

 ento-mesodermal layer, which come to lie close upon the 

 yolk. Nothing is said of the portion of the invaginated 

 cells from which these entoderm cells arise, The meso- 

 derm is later in becoming differentiated. 



Nusbaum's observations as to the place of origin of the 

 entoderm seem to aiford a partial support to those of 

 Grassi on the bee noted above ; but without illustrations 

 it is not easy to see how far this is really the case. Kow- 

 alevsky and Schulgin do not say whether the entoderm 

 arises from the whole lower surface of the invaginated cells 

 or from their anterior region. All three agree, however, 

 in saying that the yolk cells play no part in the formation 

 of the epithelium of the mesenteron. In this connection 

 my own observations ('85) on the development of Limu- 

 lus are of interest. Nothing is known of how the yolk 

 cells arise in that form, my few observations only lending 

 probability to the view that they are derivatives of the 

 early segmentation which do not migrate to the surface. 

 My sections, however, seem capable of but one Interpre- 

 tation — that the peripheral yolk cells eventually form the 

 epithelial lining ofthe liver and the intestine and that they 

 gradually devour those lying inside them, thus producing 

 the lumen of this portion of the alimentary tract. To a 

 certain extent these observations are reconcilable with 

 those just noted ; for, in Limulus, the yolk cells break up 

 and digest the yolk, but some of them beside seem to 

 be true entoderm for they form a true entodermal tissue. 



