DEVELOPMENT AND HEREDITY 1213 



rise only to the corresponding half of the body. This limitation of develop- 

 ment, however, only occurs if the intact cell be left in connection with the 

 cell that has been injured. If, in echinoderm larvae, the cells be entirely 

 separated, even as late as the eight-celled stage of division, each cell will give 

 rise to a whole embryo, differing from a normal embryo only in respect of 

 size. This difference suggests that the number of divisions that each cell can 

 undergo is predetermined in the egg-cell itself, but shows also that the cells 

 into which the egg divides are, at first at any rate, equipotential. We must 

 assume therefore that the reason why one cell under ordinary circum- 

 stances only forms one half of the embryo is that its development is regu- 

 lated and determined by the presence of the other cell in connection with 

 it forming the other half of the embryo. That is to say, the development 

 of the egg involves the reaction to environment of a protoplasm of certain 

 properties and powers of reaction. The final product of development 

 depends (1) on the nature of the protoplasm (including the nucleus) of which 

 the egg is composed, and (2) on the environmental conditions to which the 

 egg is subjected during the rapid growth and multiplication attending its 

 development. We could therefore speak of a morphology of inheritance, 

 but the morphology would be ultra- microscopic and have relation to the 

 molecular structure of the protoplasm of which the egg was composed. 



In the transmission of the potentialities of development from parent 

 to fertilised egg we must regard the nucleus as the essential structure. In 

 ordinary development the spermatozoon furnishes only a nucleus and 

 centrosome, the ovum supplying the whole of the cytoplasm. There seems, 

 however, no grounds for assigning any directive power to the latter struc- 

 ture. In echinoderm ova it is possible to get rid of the nucleus and then by 

 the introduction of spermatozoa to have an individual entirely paternal in 

 origin, which, on development, produces a larva of the paternal character. 

 In division of the egg the only part of the cell which divides so that each 

 daughter cell shall include an equal part of both parental germs is the 

 nucleus. The constant number of the chromosomes in each species, and 

 their accurate division on mitosis, suggest that the hereditary transmission 

 of the potentialities of the cell is bound up with the chromosomes. It has 

 been suggested that every character is located in a chromosome or part of 

 a chromosome. If this be the case one might regard the differentiation 

 into various tissues, which occurs in the process of development, as occa- 

 sioned by an actual loss or degeneration of the constituent parts of one or 

 more chromosomes. There is no doubt that many tissues do become thus 

 differentiated at a fairly early period in development, having undergone in 

 the process an absolute modification of their potentialities, which must be 

 at any rate shared by the chromosomes of their constituent cells. The 

 extent to which this limitation of powers of development occurs varies 

 widely in different animals. In the higher animals, such as man, epithelium 

 will reproduce epithelium, and liver-cells will reproduce liver-cells, while 

 nerve-cells are absolutely incapable of multiplication On the other hand, 

 in Crustacea a whole limb may be torn off and be regenerated from the 



