lO LILLIE. [Vol. X. 



(d) Cleavage. 

 I. The First Two Furrows. 



The unsegmented ovum adheres to the vitelline membrane 

 only in the region of the micropyle. The polar globules are 

 invariably formed just opposite to this point. This fact has 

 been commented upon by other authors and I have satisfied 

 myself with merely observing it. The significance of the fact 

 seems, however, to have escaped notice. The micropyle marks 

 the point of detachment from and, in earlier stages, the point 

 of attachment to, the ovarian wall {cf. Stauffacher, No. 59). 

 We can thus trace back the orientation of the ovum to the 

 earliest stages of its development in the ovary. The polar 

 globules cannot form at any point, but must form at one point 

 in this almost alecithal ovum. The animal and vegetative 

 poles, and therefore the relations of the ectoderm and ento- 

 derm are (normally) determined before the ovum leaves the 

 ovary. The importance of this fact in the interpretation of 

 cleavage is sufficiently obvious. 



The first segmentation plane divides the ovum into two 

 unequal portions AB and CD (PI. I, Fig. 4). The division 

 runs from the animal to the vegetative poles and is inclined at 

 an angle of 45° to the future longitudinal and transverse axes 

 of the embryo. (Text — Fig. i .) When first fully formed the 

 two cells are round and meet over a comparatively small area 

 (Fig. 4) ; but they at once begin to press against each other 

 and to flatten at the point of contact (Fig. 5). This process 

 may go on until the whole egg has assumed again nearly the 

 form of a sphere ; the only external indication of the separation 

 of the two parts being a shallow constriction where they meet. 

 But no actual fusion of the cells ever takes place, for in section 

 there is always a sharp line of division. The smaller cell AB 

 is clearer than the larger cell CD, as both Rabl and Flemming 

 have remarked ; but this difference in appearance is to be 

 ascribed rather to its smaller size and hence greater trans- 

 parency, than to any marked histological differentiation. 



Each cell contains entoderm as well as ectoderm, hence 

 Rabl's designation of animal cell {AB) and vegetative cell 

 {CD) is inapplicable (No. 25). 



