986 THE PHYSIOLOGY OF REPRODUCTION. 



the zona radiata, but so soon as one has come into contact with 

 the cytoplasm of the egg a reaction ensues in the surface layer 

 which makes it impervious to other spermatozoa. The spermato- 

 zoon consists of three essential parts, — the head, the middle piece, 

 and the tail. The last named is the organ of locomotion, and 

 after the spermatozoon enters the egg this portion atrophies and 

 disappears, probably by absorption. The head of the spermato- 

 zoon represents the nucleus, and contains the valuable chromatin 

 material. On entering the egg it moves toward the nucleus of the 

 latter, meanwhile enlarging and taking on the character of a nu- 

 cleus. The egg now contains two nuclei, — one belonging to it origi- 

 nally, the female pronucleus; one brought into it by the sperma- 

 tozoon, the male pronucleus. The two come together and fuse, 

 — superficially at least, — forming the nucleus of the fertilized egg, or 

 the segmentation nucleus. The middle piece of the spermatozoon 

 a !so enters the egg, but its exact function and fate is still a matter 

 of uncertainty. Boveri believes that it brings into the egg a 

 centrosome or material which induces the formation of a centro- 

 some in the ovum, and is, therefore, of the greatest importance in 

 initiating the actual process of cell division which begins promptly 

 after the fusion of the nuclei. In the segmentation nucleus the nor- 

 mal number of chromosomes is restored, and it is believed that in 

 the subsequent divisions of the cell to form the embryo the chromo- 

 somes are so divided that each cell gets an equal division of mater- 

 nal and paternal chromosomes, and thus shares the hereditary 

 characteristics of each parental stock. This view is represented in a 

 schematic way by Fig. 305, taken from Boveri, the maternal and 

 paternal chromosomes being indicated by different colors. Accord- 

 ing to this description, both egg and spermatozoon are incomplete 

 cells before fusion. The egg contains a nucleus and a large cell 

 body, cytoplasm, rich in nutritive material, but it lacks a centro- 

 some or the conditions necessary for the formation of an astro- 

 sphere, so that it cannot multiply. The spermatozoon has also 

 chromatin for a nucleus, and a centrosome or the material which 

 may give rise to a centrosome, but it lacks cytoplasm — that is, 

 food material for growth. It would seem that if the spermatozoon 

 could be given a quantity of cytoplasm it would proceed to develop 

 an embryo without the aid of an ovum. This experiment has, in 

 fact, been made by Boveri. Eggs of the sea-urchin were shaken 

 violently so as to break them into fragments. If now a spermato- 

 zoon entered one of these fragments, which consisted only of cyto- 

 plasm, cell multiplication began and proceeded to the formation 

 of a larva. On the other hand, it would seem to be equally evi- 

 dent that if a centrosome was present in the egg or some in- 

 fluence could be brought to bear upon it to initiate the process of 



