FACTORS IN ONTOGENY 



267 



by reagents at the various stages, enable conclusions to be 

 drawn as to matters of minute detail. 



Fig. 153, A to F, presents a series of diagrams, taken from 

 Boveri, illustrating the principal facts in the process of ferti- 

 lization. In A, the egg is represented with its clear nucleus 

 in the center, surrounded by the egg membrane. Clustered 

 around the periphery are a number of spermatozoa endeavoring 

 to find their way into the substance of the egg. On the right- 

 hand side in the figure one has penetrated the membrane and 

 is shown passing into the egg cytoplasm, which puts forth a 

 small conical prominence to meet 

 it. As soon as the head of one 

 sperm enters the egg cytoplasm a 

 new membrane is formed around 

 the egg which effectually prevents 

 the entrance of any others. The 

 head and middle piece penetrate 

 into the egg, the tail usually re- 

 maining imbedded in the mem- 

 brane, where it soon degenerates. 

 A few moments after the sperm has 

 entered, a svstem of radiations 



/ 



appears around the middle piece 

 which develops into an aster sur- 

 rounding the centrosome of the 

 sperm (B). The sperm nucleus 



swells up and rapidly increases in size, its chromatin changing 

 from the compact condition in which it is arranged in the 

 sperm head to a reticulate condition (C). The chromatin re- 

 ticulum of the egg nucleus becomes also more clearly visible. 

 Sperm aster and sperm nucleus now move in toward the 

 egg nucleus, the aster usually preceding. As the nuclei ap- 

 proach, the sperm nucleus increases still more in size until 

 it becomes indistinguishable from the egg nucleus (C). The 

 chromatin network of each now breaks up into a number 

 of chromosomes, one half of the number found in the som- 

 atic cells, and the nuclei come into contact, fusing together 

 in some cases. As in the sea urchin, Echinus, the number of 

 chromosomes is eighteen, nine would therefore be found in 

 the germ nuclei; for the sake of clearness and simplicity 

 but two are represented in the diagram, those of the sperm 



FIG. 152. Egg of the worm, Myzo- 

 stoma glabrum, being fertilized by 

 spermatozoon. (After Wheeler.) 



