Hybridization of Echinoids. 15 



Lytechinus and Tripneustes. In Cidaris the primary mesenchyme 

 cells are given off from the inner end of the archenteron (figs, h, 

 I, and J of plate 3) ; in Lytechinus and Tripneustes these cells arise from 

 the posterior wall of the blastula (figs. 3, b and c), while in the hybrids 

 they arise from the wall of the archenteron at the time this structure 

 commences to grow into the blastocoele (figs, a, b, d of plate 3; figs. 

 5, c and d). The most striking result of fertilization with foreign 

 sperm has been this change in the time and place of mesenchyme 

 formation. 



Influence of Spermatozoon on Production of Paternal Characters. 



This result gives proof of an earlier visible evidence of the influence 

 of the spermatozoon in the production of paternal characters than 

 has previously been gained. The reason for this lies wholly in the 

 nature of the material. By chance, material belonging to two visibly 

 different systems of development was obtained. 



In the discussion between Boveri and Driesch, in 1903-04, on this 

 subject, evidence on the form of the larva, the skeleton, the number 

 of chromatophores, the pigment content of the chromatophores, the 

 arrangement of the chromatophores, the number of primary mesen- 

 chyme cells, and, under certain conditions, the size of the larvae were 

 considered. With the exception of the primary mesenchyme cells, 

 these are, as events happen in early development, characters which 

 are relatively late in the time of their appearance, and yet the visible 

 differences in the material were not of sufficient value to enable 

 Boveri and Driesch to reach a conclusion in common. The reason 

 for this failure to reach such an agreement seems to lie in the fact 

 that the forms used are closely enough related to have the same system 

 of development, and any differences that could appear are compar- 

 atively minor ones. 



If we consider the role of the spermatozoon in development, we 

 find that the function of the spermatozoon may be double. It may 

 give the initial impulse to development and carry the determiners 

 for the development of paternal characters into the egg; or it may 

 give this initial impulse without exerting influence in later differen- 

 tiation. 



The mature egg contains material whose differentiation will result 

 in a new individual. Treatment with egg secretions, with a great 

 variety of reagents, or the mere penetration of the surface of an egg by 

 a spermatozoon, or by a sharply pointed instrument, may cause the 

 egg to develop parthenogenetically into a thelykaryotic individual. 

 The entrance of a spermatozoon, followed by the fusion of the sperm 

 nucleus with the egg nucleus, may cause the egg to develop into an 

 individual showing biparental characters. Usually we have thought 

 of fertilization as comprising the whole series of changes occurring 



