642 ZOOLOGY SECT. 



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 Echino- 

 derm 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. 214). 

 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 rela- 

 tively small ; the important fact appears to be that this relatively 

 small particle carries with it potential energy sufficient to effect 

 the structural changes which precede 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 development of an animal, 

 taking place unless the germinal matter or germ-plasm in which they 

 originate has a correspondingly complicated and definite structure. 



The oosperm, having the faculty of reproducing the entire animal 

 without (in many cases) any further influence emanating from the 

 parent, must contain within itself something to represent each of 

 the parts even each group of cells of the adult body. The 

 oosperm of a Frog, for example (p. 280), simple though its structure 

 appears to be, must contain potentially within itself all the char- 

 acteristics of the adult animal, and not only these, but the char- 

 acteristics of each successive stage in the formation of the tadpole 

 and its metamorphosis into the adult Frog. Attempts have been 

 made to explain how it is that the reproductive cells acquire this 

 reproductive capacity. One of the most interesting of these is a 

 theory which is termed pangenesis, the origination of which is due 

 to Darwin, According to this theory, the cells of the various parts 



