218 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



and will thrive in confinement, althou gh, as Professor Reighard remarks, the eggs 

 do not fare so well. 



The early phases of segmentation of the protoplasm and yolk correspond very 

 closely to what has been described in the lobster. The protoplasm divides, and its 

 products migrate toward a limited area of the surface of the egg, which becomes the 

 animal pole. This lies usually, if not always, somewhere between the stalk of attach- 

 ment and the opposite surface of the egg. 



In one egg which I studied three small whitish spots could be seen glistening 

 through the brown yolk. These, which were some distance apart, marked the first 

 cells to reach the surface and initiate the segmentation of the yolk. In another egg, 

 which had 27 cells visible at the surface, the shell was raised or distended over that 

 hemisphere containing the animal pole. This distention of the membrane is not, 

 however, so uniformly restricted to this part of the egg as is the case with the lobster, 

 nor were the large yolk hillocks, so characteristic of the latter, discernible here. The 

 cells are now visible to the naked eye as white dots. 



Other eggs are seen in which 40 or more cells could be detected lying rather near 

 together and covering nearly half the egg, while the rest of the surface is without 

 trace of protoplasm. 



The process of migration from depths of the yolk and division of those cells which 

 have reached or nearly reached the surface continues until the whole superficies of the 

 egg is dotted with cells, 80 or more in number. What corresponds to the animal pole 

 can now be faintly distinguished where the cells are somewhat thicker or closer 

 together. No segmentation of the superficial yolk has yet taken place, although the 

 latter is apparently raised slightly about each cell. 



As cell division proceeds, the number of surface cells becomes very greatly 

 increased until 400 or more are visible. The egg then appears to be very nearly 

 uniformly segmented at the surface, and in certain phases of "rest" has the usual 

 beaded appearance. This is a late " yolk- pyramid " stage. 



The invagination stage soon follows, and a very distinct round pit appears at the 

 surface, very much in external appearance like the corresponding phase of the lobster's 

 egg, excepting that the invaginate cavity is larger. 



An invagination of a different character sometimes occurs which is probably 

 abnormal. A round, very symmetrical depression is seen in the midst of the cells 

 corresponding to the animal pole. The depression is shallow, and at its bottom three 

 or more cells can be seen looking as if they had been pushed below the surface at this 

 point. The cells bordering this depression are sometimes arranged very uniformly. 

 A similar pit was seen in the midst of the cells of the animal pole before they had 

 spread over the entire yolk. In this case it was plainly abnormal. 



I have not yet studied all the phases of the external segmentation of Cambarus 

 by means of sections, but from what has already been seen it is clear that it follows 

 in all essential details the course of events which have been described in the lobster. 



