620 SUMMAKY OF CURRENT RESEARCHES RELATING TO 



cellular connections, the rarity of early insulation of sex-cells, all favour 

 the possibility of environmental and functional variations aflfeeting the 

 reproductive organs, and thus becoming transmissible. 



Development of Nail in Human Foetus.* — M. F. Curtis has in- 

 vestigated the development of the nail in the human foetus. It follows 

 the law that the lower extremities are always developed more slowly 

 than the upper, and the dates which are here given apply to the nail of 

 the thumb. The formation of the bed of the nail commences in the first 

 week of the third month with the appearance of the first rudiment of 

 the posterior involution ; the delimitation of the bed by a peripheral 

 groove is completed towards the middle of the third month. This last 

 is effected by a simple proliferation of epithelium ; at this period there 

 is no fibrous band of perichondrial origin which can be considered as in 

 any way the cause of the folding of the epidermis. The area thus de- 

 limited in the third month may be called the primitive bed, for it is 

 composed of two segments which are separated by a secondary groove in 

 the first week of the fifth month. The anterior segment alone undergoes 

 the dorsal displacement described by Zander, and becomes later on the 

 region of the angle of the nail, homologous with the sole of the horse. 

 The dorsal segment which alone forms the nail may, from the fifth 

 month, be called the definite bed. The superficial layer which has been 

 called the eponychiiim really exists ; it is a true stratum corneum 

 which, from the beginning of the fourth month, commences to appear at 

 the anterior extremity of the bed. It then grows from before backwards, 

 and at the end of the fourth month covers the whole surface of the bed. 

 The eponychium becomes pushed off from the middle of the bed by the 

 growth of subjacent parts towards the end of the fourth month, and its 

 two extremities alone persist ; the posterior of these forms, at this period, 

 the perionyx, while the anterior becomes a persistent thick horny layer 

 at the angle of the nail. In the second week of the fourth month a 

 group of cells is differentiated at the centre of the bed, and in the midst 

 of the mucous body which will give rise to the primitive matrix. This 

 is composed of cells with grains of keratin, but these elements disappear 

 during the ninth month. By the deposit of fresh layers the primitive 

 nail is formed after the rupture of the eponychium ; this is characterized 

 by its origin from the keratin-containing cells which disappear later on, 

 by its loose irregular structure, and its constant superficial exfoliation. 

 It is really impossible to say quite exactly when a distinct layer deserving 

 the name of a nail first appears; it is by slow and continuous sub- 

 stitution that the primitive nail displaces and replaces the eponychium. 

 The primitive nail having covered the bed extends from before back- 

 wards. While the hinder part of the matrix is displaced, the central 

 part becomes the seat of a new development, for the epithelial cells 

 change in form and become filled with fine granulations of onychogenous 

 substance. The epithelial development of the matrix carries with it the 

 production of new unequal layers, which are distinctly striated, and 

 exhibit no tendency to superficial exfoliation. They constitute the 

 definite nail, and it is, again, by a process of slow and continuous sub- 

 Btitution that the definite nail displaces and replaces the primitive nail. 

 ^ The production of an eponychium, of a primitive and of a definite 

 nail are three connected facts which succeed one another in a regular and 

 constant order. 



* Journ. de I'Anat. et de la Physiol., ssv. (" 889) pp. 125-86 (2 pis.). 



