662 ZOOLOGY 



SECT. 



of a species, especially in the early stages, are not of a sufficiently 

 strongly-marked character to permit of any definite conclusions 

 being arrived at regarding inheritance from one parent as against 

 the other. Recourse has, therefore, been had to crossing between 

 distinct species or distinct genera, or even between the members of 

 distinct classes. In such experiments the Echinoderms have proved 

 capable of affording the most convenient material, and have been 

 very largely made use of. It has been found to facilitate suc- 

 cessful crossing between distinct kinds of Echinoderms if 

 the ova experimented with are first treated by certain methods 

 which have been found to prepare the way for the process 

 of artificial parthenogenesis briefly referred to in the Intro- 

 duction (Vol. I., p. 21). The ova so treated are then shaken 

 violently in a tube, with sea-water, until they become to some 

 extent broken up. From the fragments such large pieces as are 

 found not to contain nuclei are picked out, and the sperms of the 

 second kind of Echinoderm are added to the water in which these 

 non-nucleated fragments are contained. In many cases it is found 

 as a result that a sperm enters a non-nucleated fragment and an 

 embryo becomes formed. It now has to be determined how far 

 this embryo resembles and differs from the embryos of the two 

 species from which the ovum and the sperm respectively have been 

 derived. This is always a matter attended with a considerable 

 amount of difficulty, since such embryos can rarely be reared 

 beyond the gastrula stage. But evidence seems to have been 

 obtained by means of this method that the nucleus has not a 

 monopoly in the transmission of the parental characters, since in 

 such experiments maternal features do appear in the embryo, and 

 these, in the absence of a female nucleus, must have been 

 transmitted by the agency of the cytoplasm. 



Evidence tending in the same direction has been obtained as a 

 result of experiments on the ova of Ctenophora (Vol. I., p. 217). 

 If, before these are fertilised, a definite area of the cytoplasm be 

 removed without the nucleus being interfered with in any way, 

 the embryo which develops after fertilisation presents deficiencies 

 in the organs of definite areas corresponding to the parts which 

 have been removed. 



It has been urged in connection with the question of heredity, 

 that what is transmitted from generation to generation is not so 

 much matter as energy. The quantity of matter is always 

 relatively small ; the important fact appears to be that this 

 relatively small particle carries with it potential energy sufficient 

 to effect the structural changes which pi-ecede the beginning of 

 the process of assimilation, and to at least initiate that process. 

 But we can hardly imagine a succession of complicated and very 

 definite changes of structure, such as are involved in the develop- 

 ment of an animal, taking place unless the germinal matter or 



