558 A. C. WALTON 



between coitus and insemination, more or less, therefore, of the 

 refractive body may be present at the time of union of the egg 

 and sperm. The amoeboid spermatozoon, ascending the oviduct, 

 meets the descending oocyte and enters the egg, blunt end fore- 

 most, at the first point of contact — as is to be inferred from the 

 fact that there is no apparent relation of the point of entrance 

 and the position of the egg spindle. The amoeboid method of 

 penetration and the relation to the egg membrane are well shown 

 in figure 30, where the spermatozoon is found partly within the 

 egg. It is very narrow at the point of entrance, and very much 

 enlarged just within the membrane. The dark bodies in the 

 cytoplasm of the sperm are mitochondrial granules. The cen- 

 trosome (fig. 31) occupies a depression in the side of the sperm 

 nucleus. In the case here figured, a small remnant of the re- 

 fractive body is still visible. 



The entrance of the sperm affords the necessary stimulus for the 

 completion of the process of maturation. The spermatozoon, 

 after entering, migrates to the center of the egg before beginning 

 its metamorphosis. Its cell wall then disappears (fig. 32) and its 

 cytoplasm gradually merges with that of the egg (figs. 33 to 35). 



As shown in a former article (Walton, '16 c), the mitochon- 

 drial bodies of the sperm become distributed through the cyto- 

 plasm of the egg and, apparently without further participation 

 in the process of fertilization, are lost to view. 



The centrosome (fig. 32) divides, leaving a ' centrodesmus' 

 between its halves (figs. 33 and 39), as has been shown by Bo- 

 veri and others in the case of A. megalocephala. Figure 34 

 shows the two centrosomes so far separated from each other 

 that their central connection is lost. No astral figure is apparent. 

 The separation of the halves of the divided centrosomes in this 

 case was slightly precocious, for the centrosomes ordinarily re- 

 main closely connected until after the sperm nucleus has under- 

 gone greater development than is indicated in this case. 



Figure 47 shows the nucleus lying free in the cytoplasm of the 

 egg, with the centrosomes slightly at one side, surrounded by a 

 remnant of the sperm cytoplasm. The chromatic mass of the 

 sperm nucleus, hitherto apparently homogeneous, now begins to 



