FERTILISATION OF THE EGOS OF ANIMALS. 171 



it is doubtful whether the outer hiyers of jelly are the more 

 watery and the inner the less so. In fact^ from the ease with 

 which the spermatozoa rotate the egg inside the gelatinous 

 coat (vide infra), one might well suppose that the innermost 

 layers are the more watery. The view that the resistance of 

 the jelly decreases inwards has, indeed, already been upheld 

 by Selenka^ for the eggs of Asterias. For the Echinoidea 

 and Asteroidea, therefore, the necessary basis of fact for an 

 application of Massart's theory seems to be wanting. 



There appear to me to be yet two possible explanations of 

 the penetration : (1) It is due to reaction to a stereotactic 

 stimulus; (2) it is purely mechanical. 



1. Stereotropism has long been observed. Very many 

 organisms, both animals and plants, in sea- and fresh-water, 

 grow perpendicularly to their substratum, owing to the 

 influence which the position of the latter has upon their 

 direction of growth. In the same manner as for geotropism, 

 heliotropism, chemotropism, etc., we have a corresponding 

 tactic phenomenon, so also may it be with stereotropism. It 

 is possible to imagine a free-swimming organism which, upon 

 coming in contact with a surface, receives from it a stimulus 

 which causes it to alter its movements in such a manner as to 

 attempt to make its way more or less perpendicularly to the 

 same, and through the substance concerned. Although such 

 a stereotactic sensitiveness would neatly explain the radial 

 penetration for the Echinoidea, Petromyzon, and the frog, 

 yet conclusive observations in its support appear to me to be 

 lacking. 



2. Owing to the extreme difficulty or impossibility of seeing 

 exactly what the movements of a spermatozoon upon a gela- 

 tinous surface are, the mechanical explanation must at present 

 remain tentative and almost purely hypothetical. When a 

 spermatozoon, swimming spirally, comes in contact with the 

 outer surface of the gelatinous coat, the tip of the conical 

 head, which reaches it first, possibly owing to the force of 

 contact, possibly to adhesiveness, may well be supposed to 



' Selenkuj loc. cit., II ' Die Befruclitung, Das Spennatuzooii.' 



