552 MR. J. B. SUTTON ON ATAVISM. [DeC. 21, 



investigations point to the probability that the finger typically- 

 attached in Hvpparion to the medial styloid bone was the last to 

 abort. It is certainly noteworthy that in the majority of cases of 

 Polydactyly occurring in Horses it is this digit which reappears most 

 frequently. 



In the Teratological Gallery in the Museum of the College of 

 Surgeons several specimens of Horses' feet are shown with a well- 

 developed second digit. Cheauveau figures' a specimen preserved in 

 the Veterinary Museum at Lyons ; but the most complete case of its 

 kind is that figured and described in Prof. Marsh's paper ^ "On 

 Polydactyle Horses, Recent and Extinct." In this instance the 

 inner digit was present on the four feet. Besides giving an in- 

 teresting account of extra digits noticed by early writers, this eminent 

 palseontologist tells us that the more frequent occurrences of extra 

 (iigits on the manus is precisely what a study of fossil forms of equine 

 mammals would lead us to anticipate. 



These cases support the view as Gegenbaur points out, that the 

 atavistic parts do not belong to forms palaeontologically remote or 

 systematically far distant. 



In the Spider-Monkey the thumb is normally absent, or repre- 

 sented merely by a nodule of cartilage or fibrous tissue. Yet on 

 one occasion I dissected an Ateles paniscns which had on each hand 

 a perfectly developed thum.b. This condition is not very uncommon. 

 If the parts be dissected in normal specimens, the thumb is repre- 

 sented by a band of cartilage and fibrous tissue attached to the 

 trapezium. The illustrations which have been used were selected 

 merely to give a clear notion of genuine Atavism — the palteogenetic 

 form. The question of Polydactyly is in some instances susceptible 

 of another explanation, which I do not propose to speak of in this 

 paper. 



Atavism in relation with the Prostate. 



Leaving the skeleton, attention will now be invited to a re- 

 markable instance of Atavism presented by the prostate. In struc- 

 ture and intrinsic relations this organ ])resents some exceptional 

 features. It is best regarded as a capsule composed of fibrous and 

 unstriped muscular tissue, mixed with tubular glands, surrounding 

 a recess known as the utriculus masculinus. The utriculus separates 

 the termination of the confluent ducts of the vesicuJa) seminales and 

 vasa defereutia, whilst it is tunnelled near its anterior aspect by the 

 commencement of the urethra. The glands of the prostate are best 

 considered as tubular alveoli which unite into a smaller number of 

 excretory ducts (about twenty) opening in two depressions of the 

 urethra known as the prostatic sinuses. The tubules are lined by 

 columnar epithelium. 



The central recess before mentioned has been named the sinus 



^ ' Aiiatomie Comparce.' 



* American Journal of Science and Art, toI. \m., June 1879. 



