266 GENERAL EMBRYOLOGY 



not attempt to relate the evidence directly to either hypothesis, 

 leaving that for the reader; and in conclusion we shall attempt a 

 summary which may serve to bring the two hypotheses 

 together. 



We may first describe certain facts associated chiefly with the 

 hypothesis of germinal cytoplasmic localization. 



In the first place it is perfectly clear that the ovum does 

 possess a marked structure and organization, indicated in 

 several ways. Occasionally the ovum may show external 

 differentiations of form; such an egg as that of the squid 

 (Loligo, Fig. 113) or the fly (Musca, Fig. 47), is obviously not 

 only bilaterally symmetrical but it exhibits definite antero- 

 posterior and dorso- ventral differentiation. In a few instances 

 the eggs of a species are dimorphic, and while apparently the 

 nuclei of both kinds are identical in structure, the total 

 volume of one form may be three times that of the other. One 

 of the very important and highly significant factors in the 

 organization of the ovum is that of polarity, already described 

 (e.g., Figs. 42, 93). In many cases the polarity of the ovum 

 can be traced back into oogonial stages, where it is seen to 

 correspond with the polarity of the cells of the germinal epithe- 

 lium (Mark). Polarity pervades the whole structure of the 

 mature ovum and is expressed in a variety of ways, by the 

 eccentric position of the nucleus, by the point at which the 

 polar bodies are formed, by the disposition of the deutoplasm, 

 and by the arrangement and distribution of a variety of formed 

 substances, such as pigments, granules, and vacuoles of many 

 kinds. 



Living protoplasm, as we have seen, consists of a fundamental 

 matrix or ground substance of rather uncertain form and com- 

 position, and suspended within this, particles and granules 

 of many sizes, forms, and materials, the nature of which gives 

 character to a particular region of protoplasm. These different 

 kinds of substance are not distributed at random through the 

 ovum, but they are localized in certain regions, as zones or 

 layers, either horizontal or concentric. 



There are at present two views as to the relation of these 



