DEVELOPMENT OF THE INDIVIDUAL 



571 



to understand the more important as- 

 pects of medicine we must reach into the 

 fundamental workings of development and 

 growth. This can only be done with lower 

 animals and it is through the knowledge 

 stemming from such work that the near- 

 ultimate answers are going to come. Let us 

 examine some of the more recent informa- 

 tion concerning development. 



Fertilization 



The logical place to start in an experi- 

 mental study of development is fertilization. 

 Is it necessary that a sperm enter the egg? 

 What factors influence its entrance? Aside 

 from contributing a complete set of chromo- 

 somes, what other function does the sperm 

 perform? These are some questions that 

 have invited experimentation and for which 

 we have some answers. 



Even Aristode knew that the drone bee 

 was a result of an unfertilized egg. How 

 does this fact fit into the general assumption 

 that all eggs must be fertilized? The situa- 

 tion in bees as well as other animals led 

 biologists very early to investigate the ne- 

 cessity of the sperm for development. Par- 

 thenogenetic development, as this is called, 

 can be produced in a large number of ani- 

 mals by simply subjecting the eggs to an 

 appropriate chemical or physical change, 

 such as immersing them in certain chemi- 

 cals or pricking them with a needle dipped 

 in blood (Fig. 23-9). This would seem to 

 indicate that the sperm serves two func- 

 tions: one to contribute a haploid set of 

 chromosomes, the other to initiate develop- 

 ment of the egg. Apparently the potentiali- 

 ties of development are locked up in the 

 egg, waiting to be released at the right time 

 by the entrance of the sperm. If other suit- 

 able physical or chemical factors are ap- 

 plied, they too can perform this second 

 function of the sperm very satisfactorily. 

 The action is probably tied up with en- 

 zymes in some sort of trigger mechanism 

 which can be set off by a variety of stimuli. 

 What this mechanism is will be revealed 



only when we learn more about the funda- 

 mental nature of protoplasm itself. 



Principle of potency 



What are the factors that determine 

 which parts of an embryo will produce 

 what structures? In most animals the cells 

 in early cleavage resemble one another very 



Fig. 23-9. Parthenogenetic development can be obtained 

 in a rabbit's egg by pricking it with a needle dipped 

 in blood. 



closely. At this time, say in the eight-cell 

 stage, is each of these cells assigned a spe- 

 cific job to do? Will one become muscle, 

 another nerve, and so on? This was one of 

 the initial problems investigated by early 

 embryologists. The cells of cleaving sea ur- 

 chin eggs were separated in the two-, four-, 

 eight- and 16-cell stage and their develop- 

 ment followed. It was found that in each 

 case the separated cell had the ability to 

 give rise to a complete embryo though pro- 

 gressively smaller, the last one being one- 

 sixteenth the size of a normal embryo. The 

 separation of eggs beyond the 16-cell stage 

 did not result in complete embryos. This 

 indicates that each cell retains its potency 

 to produce a complete embryo up to the 16- 

 cell stage, beyond which point it becomes 

 a part of the embryo. Such early cells are 

 said to be totipotent, that is, they retain all 

 of the potentialities of the original egg and. 



