542 William A. Hilton. 



to the axis drawn through animal and vegetable pole, shows the cells 

 on the exterior and the central undivided yolk mass. In more ad- 

 vanced stages of late morula the cells of the animal pole are numerous 

 and smaller, sections show a considerable advance over conditions 

 found in an egg like Fig. 22, The small cells of the animal pole have 

 become very numerous, quite small and of more than a single cell 

 layer in many places, while below these and separated by a slight 

 segmentation cavity, the central yolk mass is shown for the first 

 time completely divided into separate blastomeres, some of them 

 large, it is true, but the segmentation is total. The cells above the 

 small segmentation cavity are somewhat rounded in form and smaller 

 than the other cells which are composed of larger yolk granules of 

 the vegetative pole. Between and about the larger cells near the animal 

 pole are spaces continuous with the segmentation cavity. It would 

 seem that if many of these cells divided and became more compactly 

 arranged the spaces between the cells as well as the rather small 

 segmentation cavity might come to be obliterated. In later stages 

 this is what seems to take place, but in such stages very little can be 

 learned from surface views, the whole egg appears like an unsegmented 

 o^'^lm because the blastomeres are so small. In sections of a number 

 of eggs of about this stage, the whole egg seemed to be a solid mass 

 of rather small, large-granuled cells surrounded in large part by a 

 rather definite more or less simple layer of small cells with small 

 yolk granules. The extent of these small cells with the fine yolk 

 granules was found to be quite a little greater than in the last fine 

 morula stage recognized, and as a large part of this surface seemed 

 to be made up of small cells, in many places of only a single layer, 

 it seems quite probable that there is quite a migration of the small 

 cells of the animal pole over the large yolk granule cells of the vege- 

 tative pole. There is at least a considerable rearrangement of cells 

 to form this simple covering, but there is also a strong probability 

 that the large central cells contribute somewhat in the formation of 

 this small cell covering which partly surrounds the amtral mass. 



The way in which the small cells seem to grow about the rather 

 solid central yolk, seems to be unlike anything I have found described 



