512 BULLETIN OF THE 



C. THEORETICAL CONSIDERATIONS AND GENERAL 



CONCLUSIONS. 



Promorphology op the Ovum. — In the earlier stages of its growth 

 from an indifferent cell, the ovum remains for a time in the condition 

 of an homaxial body.* The polyhedral form, which so often obtains, 

 is not of fundamental significance ; it is simply the temporal phase, 

 which its surroundings mechanically impose upon the ovum. This is 

 clearly shown by the spherical form which such ova usually assume as 

 soon as they are set free. The homaxial condition of the ovum is 

 evinced by the central position of its nucleus, the germinative vesicle, 

 as well as by the form of the protoplasm which constitutes its principal 

 bulk. Whether granular or more homogeneous, the latter exhibits in 

 every radius the same conditions. 



Ultimately, however, one of its axes undergoes important changes, 

 and thus becomes differentiated from the remaining axes ; these still 

 continue in a state of mutual equality. Such may be called the monax- 

 ial condition. Although the differentiation of this axis is manifest in a 

 variety of ways, and perhaps occurs at different stages of development 

 in different animals, there are still many reasons for believing that this 

 differentiated axis is homologous throughout the Metazoa. It may properly 

 be called the primitive axis. It is that — to mention a single one of 

 its characteristics — in which the maturation spindle lies just prior to 

 the formation of the polar globule. While the spindle may possibly fur- 

 nish in some cases the first readily appreciable indication of such a con- 

 dition, it is probably only one of the evidences of a pre-existing monaxial 

 state. 



Since the differentiation of the primitive axis usually appears to be 

 coupled with a specialization of one of its poles, it remains at present 

 doubtful if a haplopolar monaxial stage intervenes between the homaxial 

 and the diplopolar monaxial conditions. I have, however, already (p. 186) 

 called attention to a condition of the egg in the development of Limax 

 which seems to realize the requirements of such a stage. The poles of 



* Even in those cases (Vertebrata) where the permanent ova are formed by a con- 

 fluence of several primitive ova, the homaxial condition ajjpears to be only tempora- 

 rily obscured, and is none the less an essential phase in the growth of the permanent 

 ovum. See Goette '75 and Balfour '78^. 



