DEVELOPMENT 199 



de- differentiation of the cells. In all these phenomena we see 

 something very strange indeed — a suggestion, in a way, that the 

 life-career of an organism from youth to old age may possibly 

 be reversed. 



71 6. Re-Differentiation. In cases like that of the parasite, 

 Saccidina, there is a new process of differentiation. The 

 embryonic cell mass that is injected into the body of the crab 

 has no organ-anlagen or tissues. Presently, however, the process 

 of organogenesis begins anew and is followed by histogenesis. 

 But both the organs and the tissues of the adult, reproductively 

 mature Sacculina that are so formed are quite different from those 

 of the Cypris-larva that de-differentiated before infecting the crab. 

 The process is very curious and is difficult to describe in terms 

 of modern conceptions of genes (Section 74^). 



72. ON EMBRYOGENY : III. DISHARMONIES AND 



REGULATIONS 



For each organism there is a normal and specific developmental 

 career and this may be remarkably constant and true to type : on 

 the other hand, the development of an animal may display extra- 

 ordinary departures from type. For instances : it would be easy 

 to study very many thousands of flounder embryos incubating 

 in a hatchery without finding a single abnormal specimen ; on 

 the other hand, malformations among the trout embryos seen in 

 a fish hatchery are not infrequent. The ordinary experience of 

 a medical man includes few cases of abnormal development : 

 nevertheless, there are very numerous records of " monstrosities " 

 of bizarre form. As in the cases of the trout embryos of abnormal 

 type such human monstrosities are very seldom viable. 



The reasons for abnormal development that introduces dis- 

 harmonies into the embryonic structure are obscure. There are 

 natural physical conditions : temperature, normality in the 

 chemical state of the nutritive medium, etc., and these have certain 

 optimal values for every development. In some cases the physical 

 conditions, say, temperature, can be widely varied without doing 

 more than retard or accelerate the rate of development — the result 

 may be a perfectly normal embryo. In other cases relatively 

 small chemical changes may be significant : thus the removal of 

 the trace of calcium that sea-water contains may lead to the 

 blastomeres of a sea-urchin embryo falling apart and living on as 



