568 A. C. WALTON 



The present paper has shown that in the ogoonia of A. canis 

 there are thirty-six tetrads — twenty-four autosomes and twelve 

 idiosomes — the diploid number. Through a pseudo-reduction 

 by parasyndesis, occurring during the prophase of the first 

 oocyte, the number of chromosomes is reduced to eighteen, all 

 di-tetrads in form. By means of the two maturation divisions, 

 the mature state is reached. But maturation does not proceed 

 beyond the formation of the first spindle, and its subsequent 

 migration to 'the periphery of the egg, unless one of the amoeboid 

 sperms has entered. As soon as a sperm has penetrated, a thick 

 homogeneously staining, highly refractive cell-wall is secreted 

 around the egg, just within the original cell-membrane. This 

 wall prevents the entrance of further spermatozoa. 



One of the female centrosomes is eliminated at the second 

 maturation division, and all trace of the other is lost. The 

 sperm carries with it a centrosome imbedded in its nucleus; and 

 as the latter breaks up into its component chromosomes, the 

 centrosome divides into halves, which separate and lie free 

 within the cytoplasm of the egg, often in a position midway be- 

 tween the two pronuclei. It is the centrosome of male origin, 

 therefore, which functions in the cleavage spindles. 



The chromosomes of the male and female pronuclei, each en- 

 closed within a nuclear membrane, become broken up into 

 minute granules arranged on a linin network. These granules 

 gather toward one pole of the nucleus in a manner somewhat 

 resembling the 'bouquet' found in the corresponding stages of 

 cell division in many other animals. From this mass the indi- 

 vidual chromosomes — either twelve or eighteen in the male pro- 

 nucleus, and eighteen in the female pronucleus — are formed, 

 and take up peripheral positions within the nucleus. 



The two pronuclei approach each other, the portion of the wall 

 of each which is adjacent to the other breaks down and either 

 their contents intermingle in a single 'fusion nucleus,' — the less 

 common method, — or the polar as well as the adjacent walls of 

 the pronuclei break down and the chromosomes, thus set free, 

 immediately become arranged in the spindle set up between the 

 halves of the divided male centrosome — the more common 

 method. 



