NUCLEI OF ANIMAL AND VEGETABLE CELLS. 145 



Fertilisation is, therefore, the fusion of the spermatic sub- 

 stance with the superficial layer of the yolk. 



After its occurrence the yolk is divisible into three zones, 

 a central finely and regularly granular, an intermediate 

 coarsely and irregularly granular, and a superficial of highly 

 refractile, punctate appearance. In the last-mentioned layer 

 a small, round, homogeneous spot — the peripheral pronucleus 

 — developes, resembling somewhat a vacuole, and not tinting 

 in perosmic acid like the surrounding substance. It increases 

 in size and obtains interior nucleolar structures. 



At the same time two or three small clear irregular bodies 

 appear in the central zone and fuse into a central pronucleus 

 of somewhat indefinite contour and knobbed or knotted surface 

 (un corps bossele a sa surface). 



The pronuclei approach and meet near the centre of 

 the ovum, the protoplasm of which exhibits a radiate 

 arrangement. One of them — the peripheral — continues to 

 grow, while the other diminishes in size ; until finally but one 

 body can be seen, of irregular and indistinct contour and con- 

 taining no nucleoli. The exact mode of formation of this 

 nucleus — Avhether by fusion of both jDronuclei, or by absorp- 

 tion of one by the other — Beneden could not clearly determine. 



The origin of the peripheral pronucleus from that layer 

 which was immediately touched by the spermatozoids gives 

 colour to an hypothesis of a fusion of male and female elements 

 in the union of the pronuclei ; but van Beneden leaves it for 

 the present among the number of mere possibilities. 



The changes which occur immediately after this stage 

 have not yet been studied by van Beneden as closely as they 

 deserve to be. Nevertheless, he saw the primary cleavage 

 nucleus elongate and a figure develop which he compares 

 with Auerbach's karyolytic figure ; and he believes, more- 

 over, that he has sufficient ground for stating that the 

 vacuoles which the latter observer describes as occurring in 

 the intermediate band are merely remains of the primary 

 embryonal nucleus, i. e. the nucleus of the primary cleavage 

 mass, and, as such, are capable of becoming tinted by treat- 

 ment with picrocarminate of ammonia. 



Immediately after completed cleavage, each segment is of 

 regular spherical shape, and presents a clear spot within, 

 which, under a high power of microscope, is seen to be 

 composed of two distinct parts — a smaller, round part, con- 

 sidered by van Beneden to be derived from the primary 

 embryonal nucleus and called by him the derived pronucleus, 

 and a larger part, with knobbed surface, incompletely 

 enveloping the former, called the produced pronucleus 



NEW SEK. VOL. XVI. K 



