194 



PHYSIOLOGY 



CHAP. 



It leaves the periphery of the yolk to approach the centre, where 

 it waits to be fertilised. 



The process of fecundation, as that of maturation through 

 which are formed the polar bodies, has been well studied in the 

 transparent ova of the echinoderms and ascarides. When the two 

 sexual elements are mixed in sea water, a great number of 

 spermatozoa soon approach the ovum, and fix themselves on the 

 gelatinous envelope. 



But one only of them normally penetrates the ovum and fertilises 

 it. At the point where it touches the ovum with the extremity of 

 the head, the hyaline protoplasm forming the cortical stratum of 

 the ovum rises into a little prominence called the cone of attraction 

 (Fig. 73), which the head of the spermatozoon penetrates with its 

 active pendular and boring movements. The spermatozoon having 



B 





Fiu. 73. Portions of ovum of Asterias glacialis during the copula externa with the spermatozoon. 

 (According to Fol.) A, the spermatozoa have already come in touch with the mucous covering 

 which invests the surface of the ovum, which begins to push out a prominence in front of the 

 spermatozoon which is nearest to it ; P>, the prominence and the head of the spermatozoon 

 have made contact ; C, the head of the spermatozoon lias penetrated into the ovum and at the 

 place of penetration the ovum has formed a vitelline membrane. 



penetrated into the ovum, the aperture is closed by the secretion 

 of a fine vitelline membrane, which prevents the entrance of a 

 second spermatozoon. 



To the copula externa of the two sexual cells succeeds the 

 copula internet, t that is the act of fertilisation properly so called. 

 The head of the spermatozoon having penetrated into the 

 ovum, grows rapidly in volume and acquires the appearance of 

 a vesicle, which is called the male pronucleus, whilst the tail 

 ceases to move and dissolving gradually soon disappears. The 

 male pronucleus leaves the peripheral zone of the ovum and 

 approaches the centre in the direction of the female pronucleus. 

 During this centripetal movement a special attraction appears to 

 be exercised on the granules of the yolk, for they dispose them- 

 selves around in the form of rays, which become always more 

 distinct and extended. While the male pronucleus is approaching 



