^^O JORDAN. [Vol. VIII. 



possible explanation of the difference in cell size ; the fact 

 that the dorsal archenteric cells are smaller than the ventral, 

 and resemble in this respect the ectoblast cells, is no proof of 

 their direct genetic connection with the latter : for aught that 

 is known to the contrary a secondary resemblance may have 

 been acquired in situ. 



The presence of pigment in the borders of the cells supposed 

 to be invaginated has likewise been taken as j^roving their 

 external origin, but this cannot be regarded as a more con- 

 clusive argument than that drawn from the size of the cells. 

 The pigment, even in early segmentation, is not confined to 

 the cells of the upper pole, but darkens to a greater or less 

 extent the borders of all the cells. At the time the archenteron 

 begins to be formed pigment is found in the cells on both sides 

 of the archenteric cleft, and as the cleft widens and deepens 

 the black borders of the lining cells become more and more 

 pronounced. Furthermore, as is well known, a double line of 

 pigmented cells extends forward beyond the limit of the actual 

 cleft, and this occurs at a stage when it is exceedingly unlikely, 

 if not impossible, that epiblast cells should have invaginated to 

 that distance. The fact that the pigment in this region is 

 found in the opposed borders of tivo rows of cells cannot be 

 regarded as strengthening the opinion that the presence of 

 pigment signifies invaginated ectoblast cells. The presence of 

 pigment in certain cells, therefore, should by no means lead us 

 to refer those cells to an ectoblastic origin. It might on the 

 contrary be quite as reasonably supposed that the pigment 

 marks physiological activity, and that the less heavily pig- 

 mented cells of the ventral wall of the archenteron owe their 

 relative lack of pigment to the more sluggish metabolism 

 attendant upon less rapid cell division. That is to say, the 

 presence of pigment may be directly correlated with the small 

 size of the cells containing it. 



The size of the yolk granules has sometimes been taken as 

 affording a criterion of invagination. The yolk granules in the 

 dorsal archenteric cells are much smaller than those in the 

 ventral, and this characteristic is thought to relate the dorsal 

 cells to the ectoblast cells and distinguish them from the cells 



