THE VITAL PROPERTIES OF THE CELL 257 



a. Echinoderm Eggs. In most Echinoderms, the minute trans- 

 parent ova are laid in sea-water, in a completely mature con- 

 dition, having already budded off the pole cells (p. 229), and 

 developed a small egg nucleus. They are surrounded by a soft 

 gelatinous sheath, which can be easily penetrated by the sper- 

 matozoa (Fig. 138 A}. 



The spermatozoa are exceptionally small, and consist, as is the 

 case in most animals, of (1) a head resembling a conical bullet ; (2) 

 a small spherule, the middle portion or neck ; and (3) a delicate, 

 contractile, thread-like tail. The head contains nuclein, the 

 middle portion paranuclein, whilst the tail consists of modified 

 protoplasm, and may be compared to a flagellum. 



If ova and spermatozoa are brought together in sea- water, several 

 of the latter immediately attach themselves to the gelatinous 

 envelope of each ovum. Of these, however, only one normally 

 fertilises each egg, namely, that one which, by means of the 

 undulating movements of its tail, was the first to approach its 

 surface (Fig. 138 A-G). At the spot where the apex of the head 

 impinged, the hyaline protoplasm constituting the peripheral 

 layer of the ovum raises itself up to form a small protuberance, 

 the receptive protuberance. Here the head, impelled by the 

 undulating movements of the tail, bores its way into the ovum, 

 which at this moment, excited by the stimulus, excretes a deli- 

 cate membrane, the vitelline membrane, upon its surface (Fig. 

 138 (7), and, apparently by means of the contraction of its contents, 

 presses some fluid out of the yolk. In consequence, a gradually 

 increasing intervening space, which commences at the receptive 

 protuberance, develops between the yolk and the yolk mem- 

 brane. By this means the entrance of another spermatozoon is 

 prevented. 



Processes occurring in the interior of the yolk follow the external 

 union of the two cells ; these may be grouped together under the 

 common name of internal fertilisation. 



The tail ceases to move, and soon disappears from view ; the 

 head, however, slowly pushes its way into the yolk (Fig. 139 A) ; 

 meanwhile, it absorbs fluid (Fig. 139 B), and swells up to form a 

 small vesicle, which may be called the sperm-nucleus, or male 

 pro-nucleus, since its essential constituent is the nuclein of the 

 head of the spermatozoon ; hence it becomes intensely stained by 

 carmine, etc. Fol has lately shown that immediately in front of 

 it, on the side which is directed to the centre of the egg (Fig. 



s 



