EGG OF ALLOLOBOPHORA FOETIDA. 55 



in the cytoplasmic network and to represent transverse sections 

 of the cytoplasmic threads. 



This Qgg might be subpoenaed as a witness for the theory 

 that the centrosome is a mechanical centre which appears only 

 during the so-called active stages of development. The &gg 

 attraction sphere is present during the two maturation divisions; 

 but after the second polar body is formed and the female pro- 

 nucleus begins to develop (Fig. 5), it totally disappears. The 

 sperm attraction sphere is present until the head of the sper- 

 matozoon begins to develop into the male pronucleus and then 

 it also totally disappears (Fig. 5). Both spheres are absent 

 during a relatively long period (that is, while the young pronuclei 

 are developing), and when the pronuclei have attained their 

 maximum size, two attraction spheres appear again in the cyto- 

 plasm, and the cleavage spindle is formed. 



If we believe that both attraction spheres are cytoplasmic 

 phenomena, that the constituent parts of each are made up of 

 the same cytoplasmic elements (that is, that they are alike mor- 

 phologically), and that they differ only in that different condi- 

 tions are necessary to the appearance of each, two questions 

 suggest themselves: First, if the two spheres are alike, why 

 does a later stage of development of the o.^^ seem to be neces- 

 sary to the appearance of the male attraction sphere, why does 

 the latter not appear as early as do the first maturation spheres ? 

 It seems, however, that it does not (and this is true not alone 

 of this egg). Does this indicate a difference in the attraction 

 spheres themselves, or does it indicate a definite change in the 

 cytoplasm } 



In this connection it is interesting to note that the structure 

 of the cytoplasm before the polar bodies are formed differs 

 somewhat from its structure after the polar bodies are formed. 

 At the earlier stage the meshes of the network are very much 

 closer and at the later stage they are more open (alveolar) in 

 structure, this condition bearing a definite relation to the periods 

 of rest between the divisions and reaching its acme at the pro- 

 nuclear stage just before the cleavage attraction spheres appear. 



Another puzzling question suggests itself at least to a prac- 

 tical observer — a question of economy. If the two spheres 



