240 



INTRODUCTION TO CYTOLOGY 



(annelid) and Ascaris (nematode) penetration takes place before the first 

 mitosis^" (Fig. 154). 



Probably in most animals the whole spermatozoon enters the egg (Fig. 

 145). In some sea urchins only the head and middle piece enter, while in 

 Nereis the head alone passes in, leaving the middle piece and tail on the 

 egg surface. The details of penetration in two of the best known cases 

 are as follows. 



In Nereis the egg has a vitelline membrane, an alveolar cortical layer, 

 many yolk and oil droplets, and a large central germinal vesicle (nucleus). 

 If many spermatozoa are present in the vicinity, a large number attach 

 themselves to the egg, but usually all but one are carried away by an 



outflow of jelly from the alveoles of the 

 cortical layer. This layer now takes 

 the form of a zone traversed by radial 

 protoplasmic plates representing the 

 walls of the alveoles. A transparent 

 "fertilization cone" extends from the 

 inner region of the egg across this zone 

 and touches the membrane at the point 

 where the spermatozoon is beginning to 

 penetrate. The perforatorium goes 

 through the egg membrane and 

 becomes attached to the transparent 

 cone. The latter is now withdrawn, 

 carrying the head of the spermatozoon 

 into the egg with it. Thus it appears 

 that the initiative for the final act of 

 penetration lies with the egg rather than with the spermatozoon. Since 

 only the head enters the egg, it seems that the only necessary portion of 

 the spermatozoon in the actual union is the nucleus; the middle piece and 

 tail are accessory and function only as locomotor organs (F. R. Lillie, 

 1912, 1919). 



In the starfish (Fig. 146) the blunt-headed spermatozoa swim about 

 with spasmodic movements and apparently by chance come in contact 

 with the thick zone of sticky jelly surrounding the egg. The egg then 

 responds to the presence of the spermatozoa by forming one or more 

 hyaline conical projections on its surface. From the summit of each cone 

 a delicate filament grows outward through the jelly until it touches and 

 adheres to any sperm head (now motionless) which happens to lie in its 

 path. The filament then retracts, drawing the spermatozoon inward 

 through the jelly to the summit of the cone. Other filaments are at the 

 same time withdrawn whether they have secured spermatozoa or not. 



30 See Wilson (1925, p. 397) and literature cited; also McKay (1927) and Crabb 



(19276). -■ ' '.' 



Fig. 145. — Syngamy in Physa (snail). 

 Sperm head and amphiaster at right; 

 long flagellum extending toward left; 

 second meiotic division in progress. 

 {After Kostanecki and Wierzyski, 1896.) 



