REPRODUCTION. 849 



cells are very lucidly represented by Boveri in the two schematic 

 figures (Figs. 382 and 384). 



The process of maturation is also called reduction division, 

 because it is known at present that the quantity of chromatin sub- 

 stance in the nucleus of either of the sexual cells, or the number of 

 chromosomes which the chromatin thread forms, is reduced to one- 

 half of the quantity typical to all other cells of the same animal. 

 In regard to other essential parts, we find the spermatozoon contain- 

 ing only very little cytoplasm, while the mature ovum contains nearly 

 all the cytoplasm of the original ovarian ovum. The centrosome 

 is considered by Boveri to become lost in the ovum, while in the 

 spermatozoon it is retained, and later plays an important part. The 

 process of fertilization itself consists in a union of the male with 

 the female element to form one capable of being the foundation for 

 a new individual, although in regard to the details of this process 

 the facts known at present, and the interpretations given, can by 

 no means be considered conclusive. A very widely and favorably 

 accepted view is presented by Boveri in a to g (Fig. 386), and the 

 essential points are the following: either attracted by chemotaxic 

 force radiating from the ovum, or by their own locomotion, the sper- 

 matozoa come in contact with the ovum and pierce the zona radiata; 

 but as soon as one spermatozoon penetrates into the cytoplasm of the 

 ovum, a reaction on its surface takes place, making it impermeable for 

 other spermatozoa. During its entrance into the ovum, the spermato- 

 zoon usually loses the tail, while the head, which in reality repre- 

 sents the chromatin substance of the nucleus, becomes expanded, 

 takes on the character of a nucleus, and moves towards the nucleus 

 of the ovum. The egg at this stage obviously contains two nuclei; 

 the one is called male pronucleus, and the other female pronucleus. 

 Gradually both come in contact and form the so-called segmen- 

 tation nucleus. The middle piece of the spermatozoon also enters 

 the ovum. Soon, however, it reveals itself as a centrosome and acts 

 as a dynamic force for a cleavage of the segmentation nucleus, which 

 inaugurates the process of cell-division. With this first cleavage the 

 formation of a new individual has actually begun. Through succes- 

 sive cell-divisions an aggregation of cells is finally formed, which, 

 depending on the amount of nutritive material stored up in the ovum 

 for future purposes, becomes arranged either in form of a spherical 

 mass (morula), which gradually becomes hollow and is then called a 

 llastula, or as a circular disk, and in either case a uniform layer 

 of cells is gradually formed, which is known as the Uastodtrm. 



54 



