CHANGES IN THE OVUM. 



93 



paratus, are developed by the epidermic and middle layer of the blastoderm. 

 The cutaneous laminae, by the modification of their elements, form the 

 derm^ in which vessels are readily seen after three months. In the epiderm 

 it is not long before the mucous and horny layers can be distinguished ; 

 in the first, pigment cells are observed at the commencement of the fifth 

 month in the larger quadrupeds. The epiderm is easily detached from 

 the derm ; it forms a peculiar, white, completely-enveloping pellicle on the 

 surface of the latter, apparently separated from it by the growing hairs. 

 Frequently we find the integument covered by a special coating that looks 

 like varnish (the varnix caseosd) ; this would appear to be intended to 

 protect the epidermic epithelium from the destructive solvent action of 

 the alkaline amniotic fluid. It is very abundant on the human foetus at 

 all periods, but is not found on that of animals so long as the skin is gla- 

 brous. As soon as the hairs begin to appear, the epiderm is partially de- 

 tached in the form of the thin pellicle just alluded to, and the decomposi- 

 tion of which gives rise to an appearance like varnish. It is best seen in 

 the foetal Pig, the hairs on the skin appearing all at once over the 

 body; in the other domesticated animals they are only developed success- 

 ively, and consequently the shedding of the epiderm occurs partially and 

 in patches, which are insensibly confounded with the normal epidermic 

 layers. Microscopically, these flakes off^er the same characters as epider- 

 mis removed by a vesicant ; the points where the hairs have passed appear 

 as regular infundibuliform openings. When the foetus increases in volume, 

 the epiderm desquamates and the debris floats in the amniotic fluid. 



In the third month, the hairs are perceptible on the foetus of the Mare 

 and Cow. Hair follicles have been observed in embryos of the Pig which 

 did not measure more than two inches in length. They first appear about 

 the eyebrows, lips, and joints of the limbs, and the whole of the body is 

 covered at the sixth or seventh month ; they are usually observed in the 

 foetus of the Mare and Cow, around the lips, towards the eighteenth week 

 of gestation. The hair maybe shed and renewed before birth ; for it has' 

 been found in the amniotic fluid and in the stomach of the foetus. Each 

 hair is developed in a prolongation of the epidermic layer which is im- 

 bedded in the substance of the derm : which prolongation is constituted 

 by a bottle-shaped mass of cells. In the centre the cells are modified 

 and heaped up, so as to form a small cone whose base covers the grow- 

 ing papilla ; this cone elongates, until it touches the superficies of the 

 epidermis, when it becomes bent in the effort to push itself through ; but 

 finally it issues beyond the surface, where it may grow freely. 



The sebaceous and perspiratory glands are developed in a similar man- 

 ner, at the middle period of uterine life. The horny productions, such as 

 the claws, hoofs, ergots, and chesnicts, are apparent at an early stage. To- 

 wards the end of the second month there can be perceived in the foetus 

 of the Cow, at the extremity of each digit, a small, pale, and transparent 

 conical tubercle ; this is the rudiment of the claw. The hoofs of solipeds 

 appear towards the twelfth week, and about the commencement of the 

 fourth month they are more developed ; their texture has become firm and 

 opaque, whereas before it was gelatinous and transparent, and has assumed 

 a fine yellow tint. They are always soft, however, until birth, in order to 

 guarantee the integrity of the foetal envelopes. At mid-term, brown or 

 black patches appear in it, if the coronet is provided with pigmentary 

 stains ; but it is not until about the end of gestation that the horn begins 

 to show the greenish tint proper to it when destitute of pigment ; though 



