THE HORMONES IN HUMAN REPRODUCTION 



the production of a perfect animal, and needs only to be 

 given a start/ 



The eggs of mammals, including the human species, prob- 

 ably do not differ in this respect from those of sea urchins 

 and starfish. Because they are far harder to get at and less 

 resistant to handling and exposure, experimental study has 

 not progressed very far. In very recent years, Gregory 

 Pincus and his associates at Clark University have worked 

 with rabbits, using an experimental method in which the un- 

 fertilized egg was subjected to drastic cooling while passing 

 through the oviduct (Fallopian tube). A few such eggs, sub- 

 sequently replanted into other rabbits, are said to have 

 formed embryos, to have been born, and to have grown to 

 normal adult life. H. Shapiro of Philadelphia has still more 

 recently reported starting development of the rabbit's egg by 

 drastic refrigeration of the whole body of the female rabbit, 

 but up to the present none of these eggs has developed beyond 

 the earliest embryonic stages. 



I hasten to add that under ordinary circumstances, when 

 there is no meddling by an experimenter, mammalian eggs 

 live in a perfectly conditioned environment. The temperature 

 and all other conditions to which they are subjected in the 



1 In a brief chapter like this, in which I am deliberately selecting those 

 features of the natural history of reproduction which best lead up to the 

 higher animals, it is not possible to follow out all the ramifications of 

 the subject. Life processes are so richly varied that every general state- 

 ment calls for a bill of exceptions. There are, for example, many animals 

 that can produce parthenogenetic eggs, i.e. eggs that develop spontane- 

 ously without fertilization. This is the case in a great many insects. In 

 some of these instances, no doubt, a stimulus akin to that of fertilization 

 is furnished by natural conditions, such as high temperatures, desicca- 

 tion, or chemical changes within the egg, but in others there is no known 

 special stimulus. Indeed, when we reflect that a tendency to propagate by 

 division is innate in almost all animal cells, the wonder is that in most 

 species the eggs do have to be stimulated in order to develop. 



Incidentally, even in the insects and other animals with parthenogenetic 

 generations, sexual reproduction always occurs from time to time to re- 

 juvenate the line and start new clones, just as in one-celled animals. In 

 all vertebrate animals sexual reproduction is obligatory. 



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