HOMOLOGY, ANALOGY AND PLASIS 



161 



that the effective part of the gonad, in the 

 case of the male at least, is, not the 

 spermatogenic tissue, but the interstitial 

 cells or so-called "interstitial gland." 

 Obviously the Anlage or embryonic repre- 

 sentative of, say, the prostate gland of the 

 young male animal does not of itself 

 possess the power of developing to com- 

 pletion; for the attainment of full struc- 

 tural maturity a "hormone" derived from 

 the interstitial gland is necessary. 



In most cases of development no such 

 correlation of separate parts (one primary, 

 the other secondary) has as yet been 

 verified. At an early and quite undiffer- 

 entiated stage of development of the 

 respective parts, Braus removed the little 

 outgrowth or bud which represents the 

 hind-limb of a tadpole and inserted upon 

 the wound of the trunk thus caused the bud 

 of a fore-limb. In its abnormal situation 

 the latter grew and developed into a 

 typical and full-sized fore-limb, in which 

 not only the various bones, muscles, 

 joints and ligaments were complete, but 

 in which the orientation of the mature 

 transplant, as regards preaxial and post- 

 axial borders, depended upon the original 

 orientation of the graft. In this case, 

 selected as a type of the usual experimental 

 finding, the Anlage possesses the power 

 of "self-differentiation," to use the lan- 

 guage of Roux — that is to say, it grows 

 and continues to unfold by its own 

 inherent impulse. In the development of 

 the prostate gland, on the other hand, we 

 have a case of "dependent differentiation," 

 in which the power of unfolding to 

 completion does not wholly reside within 

 the Anlage. In the differentiation of the 

 prostate a moulding influence, specifically 

 hormonic and external to the organ itself, 

 comes into play. 



In respect of the secondary sexual 

 characters, castration, whether of a male 

 or of a female vertebrate, produces a 



common "neutral" or "indifferent" type 

 of animal, and Steinach has shown that 

 by experimental interchange of gonads it 

 is possible to "masculinize" an originally 

 female and to "feminize" an originally 

 male animal. Thus castration and im- 

 plantation of an ovary into a male guinea- 

 pig lead to marked growth and differ- 

 entiation of the mammary glands, while 

 implantation of a testis into a spayed 

 female leads to a penile enlargement of 

 the clitoris. Lipschiitz (1919) in a special 

 investigation of the last-mentioned change 

 finds that corpora cavernosa make their 

 appearance in the clitoris of the inverted 

 guinea-pig and also special horny spikes 

 characteristic of the guinea-pig penis. 

 In the formation of these corpora caver- 

 nosa and horny spikes we have an experi- 

 mental case of moulding by the environ- 

 ment, for in the absence of the hormone 

 from the male interstitial gland they do 

 not develop, the clitoris remaining a 

 clitoris. As establishing actual homo- 

 plasy with the corresponding structures 

 of the male penis, Lipschiitz (19x4) has 

 also shown that the development of the 

 horny spikes in the penis of a male guinea- 

 pig depends upon the testicular hormone. 

 Other cases of homoplasy have also been 

 induced experimentally. In the develop- 

 ment of the vertebrate eye it has long been 

 known that the essential parts arise from 

 two separate Anlagen; the light-recipient 

 apparatus or retina as a hollow protrusion 

 of the brain, which, becoming invaginated, 

 forms the optic vesicle; the main light- 

 refracting apparatus or lens as an in- 

 growth of the skin. The development 

 of the optic vesicle proceeds by self- 

 differentiation; when it is dissociated 

 from any connection with the brain it 

 forms a complete retina (W. H. Lewis, 

 Spemann). The development of the lens 

 is a case of dependent differentiation; in 

 the absence of the optic vesicle it fails to 





