296 LECTUKES ON BIOLOGY 



the spermatozoon penetrates, head forward. Almost at the same 

 moment the egg-plasm becomes rigid, forming a firm cover, the 

 yolk-membrane, which offers an insuperable obstacle to the 

 entrance of other spermatozoa. This is Nature's way of prevent- 

 ing the injurious consequences of over-fertilization, or polyspermy. 



Let us now observe, with the aid of the illustrations, the 

 subsequent stages of the fertilization act in Ascaris megalo- 

 cephaia. As soon as the spermatozoon has entered the ovum it 

 turns round so that its central part points to the centre of the 

 ovum. Presently there appears in this central part, which, as 

 we know, corresponds to the centrosome, the familiar radiating 

 figure. Nearer and nearer approach the sperm-head and the 

 egg-nucleus. Consisting at first only of a compact mass of 

 chromatin, the head of the spermatozoon meanwhile becomes 

 enlarged by absorbing fluid, and changes into a typical resting 

 nucleus in which the chromatin lies distributed in the form of a 

 fine net. In the meantime the paternal centrosome has divided 

 and the two new centrosomes have taken up a position on the 

 two opposite points in the egg-plasm. Between them the 

 maternal and the paternal nucleus lie now close together. 



Fertilization is now practically at an end, and the second act 

 of the drama commences, the division of the impregnated egg- 

 cell, or, in other words, its development. A direct fusion of egg- 

 nucleus and sperm-nucleus into the segmentation-nucleus can 

 take place in many cases, but there is no necessity. As a rule 

 we may assume that the processes take place as shown in our 

 illustration. We see there that now the egg proceeds to fission. 

 In the maternal and paternal nucleus there appear again two 

 chromosomes each. The nucleus membranes disappear and the 

 four nuclear loops arrange themselves in a uniform ' equatorial 

 plane.' Now takes place the familiar longitudinal fission of the 

 chromosomes and the formation of the daughter-planes. At last 

 the division is complete, and each of the two young embryonic 

 cells has once more received four nucleus-loops, of which one half 

 comes from the father, the other from the mother. During the 

 later stages of development division always takes place in exactly 

 the same manner, so that the ' hereditary substance ' for as 

 such we regard the chromatin due to each body-cell of the 

 developed animal is derived from both parents in the same 



