104 PROCEEDINGS OF THE ACADEMY OF [1888. 



secretion and sweat conjoined with elevation of temperature appear 

 to explain the retention of hair at the pudenda and axilla. Dr. Geo. 

 Dimmick of Cambridge, Mass., has informed me that he has iuflu- 

 -enced the arrangement of color marks on the elytra of Coceinella 

 by varying the temjoerature to which the insects had been subjected. 

 According to H. Pryer^ "temperature has a great evolutionary value 

 in insects." 



That margins of nutritive regions afford the conditions favorable 

 to the appearance of warts agrees with what is known in a osseous 

 system Avith resi:)ect to erostosis and absorption. In a growing 

 cranial bone I have found its greatest thickness in the position of 

 its centre of ossific deposit ; in the adult bone the thickness is great- 

 est at the margins. When sutures are well defined vascular activity 

 is most marked along their lines. In atrophy an area of deficiency 

 always occurs lying at a point somewhere between the centre of 

 , ossification and the borders of the bone.^ 



It is probably in obedience to the same law that in baldness a 

 lock of hair commonly persists at the bregma and in the upper part 

 of the metopic line. With respect to skin folds it must he said that 

 the disposition is caused primarily by the jDosition of the skeleton of 

 the limbs to that of the trunk, head and neck. In Rhinoceros and 

 Armadillo the folds answer pretty exactly to the divisions above 

 named. But the folds on the side of the trunk between the limbs in 

 Armadillo appear to be caused by muscular action if one can 

 accept the conclusions drawn from the appearances seen in the in- 

 stantaneous photographs of the hog as taken by j\Ir. IMuybridge.^ 



If motion can originate skin folds it can also determine color- regions, 

 and the category of the pigment patches in the intervals between 

 muscle-masses and the limitation of color-areas to muscle sheets be- 

 come practicable.* 



The history of each mammalian embryo must present many phases 

 of nutrition — especially of precocity and of retardation — which de- 

 termine individuality. In a litter of two or more individuals the 

 changes due to temperature, to motion, to rate of local blood iuter- 



1 Trans. Ent. Soc. Lond. 1882, 489. 



2 Am. Journ. Med. Sci. 1870, 405. 



•* Photographs issued under the auspices of the University of Penna, series 673. 



i It is a tempting subject for study to elucidate the distribution of skin 

 diseases I)y the application of the same methods undertaken in this essay. 

 The margins of the areas of the lanugo, — the course of distribution of nerves or of 

 vessels, the influence of the bone lying in close juxtaposition to the skin, the 



