BIOLOGICAL LECTURES. 



which had reached the stage represented in Fig. 5, one cell, 

 that designated pr in the figure, was found to show an abnor- 

 mal karyokinetic spindle, a triaster, indicating a simultaneous 

 division of the nucleus into three parts, being present, and in 

 the corresponding cell of the other half of the blastoderm, //, 

 the same abnormality occurred. This is simply one out of 

 several cases which could be given and may suffice to illustrate 

 what is meant by segment attraction. This name suggests an 

 interaction of certain cells of the blastoderm upon one another, 

 a sort of telepathy as it were, but what the nature of the inter- 

 action may be, indeed, whether it is in reality an interaction 

 in the strict sense of the term or not, it is quite beyond our 

 present power to determine. 



There has been a tendency, most marked, perhaps, among 

 the experimental embryologists to reduce the direction of the 

 cleavage planes, that is, the direction of the karyokinetic 

 spindles, to the action of causes acting from without the cell, 

 pressure and gravity being the forces most frequently brought 

 forward as explanations. Let us consider what takes place in 

 cases where there is apparently no chance for the operation of 

 pressure. Such a case is offered by the developing ova of the 

 Isopod Crustacea, in which the segmentation follows the typical 

 centrolecithal method. The egg of the marine Asellid Jaera, 

 to use this form for illustration, shows very near the center the 

 nucleus surrounded by a stellate mass of cytoplasm, and upon 

 the outside there is a thin layer of protoplasm, which close 

 observation will show to be united with the central nucleus- 

 containing mass by a delicate network of protoplasmic fila- 

 ments, the granules of yolk, which are quite abundant, lying 

 in the meshes of the network. When this egg segments one 

 finds that the division of the nucleus is accompanied by the 

 division of the central mass of protoplasm only, the peripheral 

 protoplasm and the network showing no indication of the 

 cleavage. And this process may be repeated again and again, 

 so that eventually there will be found distributed through the 

 mass of the ovum sixteen nuclei, each with a distinct mass of 

 cytoplasm surrounding it, every one of these masses being 

 united with its fellows and with the undivided peripheral pro- 



