140 



BIOLOGICAL LECTURES. 



in each stage of the cleavage the nuclei assume a position 

 nearer the surface and further away from the central position 

 which the original segmentation nucleus occupied. We can 

 speak of a centrifugal migration taking place, and incidentally 

 it may be pointed out how inappropriate are both the terms 

 centrolecithal and superficial which are usually applied to this 

 method of cleavage. In the earliest stage it is the nucleus 

 and its protoplasm which is central, and it is only in the later 

 stages that the cleavage can be said to be superficial, and that 

 the ovum can strictly be termed centrolecithal. 



Thirdly, and this is an important point, the ovum is still a 

 syncytium. In surface views one can readily see, especially in 

 connection with the ectoderm cells, rays extending from the 

 protoplasm surrounding each nucleus towards the lines which 

 mark the cell boundaries, and in many cases the rays of adjoin- 

 ing cells meet at the cell boundary, strongly suggesting a pro- 

 toplasmic continuity. That this actually occurs is clearly 

 shown by sections, which reveal the nature of the lines which 

 form the cell boundaries. Thus, in one of the mesendoderm 

 cells (me) of Fig. 9 one can readily perceive the rays extend- 

 ing from the protoplasmic mass to the boundary of the cell, 

 from which again rays extend further into the central yolk 

 mass. The cell boundary cuts off only a slight amount of the 

 superficial yolk, the greater portion of this constituent of the 

 ovum being destitute of protoplasm except for the scattered 

 filaments which extend into it from the cell boundary, and the 

 fact of the occurrence of these filaments, as well as the stain- 

 ing properties of the boundary wall, show that it is proto- 

 plasmic in character. It seems certain, when the results ob- 

 tained from surface views and from sections are combined, that 

 every cell of the ovum is organically united with its neighbors, 

 and that the entire ovum is a syncytium. 



As stated, this is a highly important fact, since we see from 

 it that a separation of the protoplasm into distinct spherules, 

 such as presumably occurs in cases of total segmentation, is 

 not necessary in order that histological differentiation may 

 occur. Indeed, such an idea might have been derived from 

 what we know of forms like the Infusoria, in which, notwith- 



