REPRODUCTION 



137 



University of Chicago. When the reproductive tract of a fetal rat is 

 dissected out and grown in tissue culture, development occurs normally 

 if the testis or ovary is left in place. If both testes are removed, there is 

 no development and differentiation of the accessory organs— vas deferens, 

 seminal vesicles and prostate gland. However, if both testes are removed 

 and a pellet of testosterone, the male sex hormone, is implanted, de- 

 velopment proceeds normally. This shows that testosterone is a morpho- 

 genetic substance which can diffuse across a limited space and induce 

 the development of male characters. 



Evidence that chemical differentiation precedes morphologic differ- 

 entiation of a tissue has come from research using serologic and bio- 

 chemical methods. The specific protein of the lens of the eye can be 

 detected serologically in the chick embryo before the lens vesicle closes 

 and before there is any evidence of morphologic differentiation. Cholin- 

 esterase is the enzyme which hydrolyzes acetylcholine and is believed to 

 play an important role in the transmission of the nerve impulse. Edgar 

 Boell of Yale has shown by microchemical methods that the neural folds 

 of the frog embryo, the parts which will form the central nervous system, 

 have much more cholinesterase than the epidermis does. ^Vhen epidermis 

 is stimulated to form nervous system, by grafting a piece of chorda- 

 mesoderm beneath it, the tissue becomes rich in cholinesterase. 



In view of the extreme complexity of the developmental process it 

 is indeed remarkable that it occurs so regularly and that so few mal- 

 formations occur. About one child in one hundred is born with some 

 major defect, a cleft palate, club foot, sjjina bifida or the like. Some of 

 these are inherited and others result from environmental factors. Experi- 

 ments with fruit flies, frogs and mice have shown that x-rays, ultraviolet 

 rays, temperature changes and a variety of chemical substances will 

 induce alterations in development. The kind of defect produced is a 

 function of the time in the course of development at which the environ- 

 mental agent is applied, and does not depend to any great extent on the 

 kind of agent used. For example, x-rays, the administration of cortisone 



Neural folds 

 Primary embryo 



NcuraJ. 

 Polds 



Neural -fcuic- 

 Optic ves^el- 



Ta.ii (Znd — ' 



O to cyst 



-Scconda-iy 



zmbr yo 



Somites of 

 secondcLry <Z-3xihryo- 



D 



A B C 



Figure 6.13. The induction of a second frog embryo by the implantation of the 

 dorsal lip of the blastopore from embryo A onto the belly region of embryo B. Embryo B 

 then develops through stage C to a double embryo, D. 



