BE PR OD UCTION. 467 



bavins: come, however, from the mule cell and one-half from the female cell. 

 On the commonly accepted theory that they constitute the hereditary .-ub- 

 stance, the first segmentation-nucleus contains within itself potentially all the 

 inherited qualities of the future individual. 



While the head of the spermatozoon is making its way through the sub- 

 stance of the egg there appears beside it a minute cytoplasmic body, the 

 centrosame, and around the latter cytoplasmic filaments arrange themselves 

 in the form of a star, the whole body being known as the sperm-aster (Fig. 

 223, b). We have previously recognized such a structure in the ovum at the 

 time of maturation, and have found it functional in the formation of the 

 polar bodies; after maturation it disappears. The sperm-aster accompanies 

 the sperm-nucleus, becomes gradually enlarged, and finally comes to lie, a 

 large and prominent body, beside the segmentation-nucleus. The origin of 

 its centrosome has been greatly disputed. Some investigators maintain that 



Fig. 223.— Stages in the fertilization of the egg (after Wilson). The drawings were made from sections 

 of the eggs of the sea-urchin, Toxopneustes variegatus, Ag. 



a. The surface of the egg has become elevated to form e, the entrance-cone for the spermatozoon ; the 

 head (ft) and the middle-piece (m) of the latter have entered the egg. 



B. Five minutes after entrance of the spermatozoon. The head, now the male pronucleus, has rotated 

 180 degrees, and has travelled deeper into the ovum. The cytoplasm of the latter has become arranged 

 in a radiate manner about the middle-piece of the spermatozoon, now the centrosome, to form the sperm- 

 aster; the egg-nucleus, now the female pronucleus, is approaching the sperm-nucleus; its chromatin 

 forms an irregular reticulum. 



it is formed anew in the egg; but the prevalent opinion at present seems to 

 be that it comes from the spermatozoon in immediate relation to the middle- 

 piece, and hence is exclusively of male origin. 



There results from fertilization, it is perceived, a single cell complete in 

 all its essential parts. This is the starting-point of the new individual. A 

 pause or resting period usually follows fertilization, ami then growth begins. 



Segmentation. — The process of growth is a complex process of repealed 

 cell-division, increase in bulk, morphological differentiation, and physiological 

 division of labor. 



Cell-division is largely, if not wholly, indirect or mitotic. The term seg- 

 mentation, or cleavage, of the ovum is conveniently applied to the 6rs1 few 

 divisions, although the details of segmentation are not different fundamen- 

 tally from those manifested later in the division of more specialized cells. 

 Each division may be resolved into three definite acts, which, however, 

 overlap each other in time. The first act is characterized by the appearance 

 of two centrosomes, each with its astral rays, in place of the one already 



