PERIODS OF DEFELOl'MKXT. 117 



at the moment of impregnation, and its containing within itself alone 

 the elements upon which the primary steps of evokition depend, there 

 being at tirst no direct connection between the embryo and the uterine 

 vessels. The parts which in reality grow most rapidly in the early 

 period of gestation, are the enveloping membranes and the Uuids they 

 contain. In the Sheep, towards the end of the lirst quarter of gesta- 

 tion, the membranes alone weigh from live to nine times more than the 

 foetus ; while the allantoic and amniotic fluids are from eight to ten 

 times as heavy. These fluids increase in an absolute manner from the 

 commencement, until within perhaps a short time of parturition. Rapid 

 at first, towards the middle period their secretion abates, and then begins 

 the more active development of the fcrtus, with a relative diminution in 

 the proportion of the fluids ; until about the end of the fourth month 

 they only represent something like a third of its weight. In the Sheep 

 a few days after the fifth month, or about birth time, they are a sixth 

 of the weight of the young animal. 



The development of the foetus is so tardy at first, that the fcetal 

 Lamb at the fortieth day has scarcely acquired the 300th part of the 

 weight it has at birth ; at two mouths it has only a 60th, but now it 

 grows more rapidly ; so that at the expiration of the fourth month it 

 has gained more than one-third of its total weight. 



The inequality in the growth of the different organs is also very 

 notable ; those whose functions are most required, such as the nervous 

 centres, liver, etc., attaining considerable proportions very quickly. 

 The brain, for instance, soon acquires a marked predominance over the 

 other organs. In the fcctus of the Lamb at the lifty-seventh day, the 

 proportion of the entire brain to that of the body in weight is as 1 to 

 15; at the seventieth day it is as 1 to 55 ; and at birth as 1 to 65. At 

 mid-term with the Calf, its weight is to that of the body as 1 to 53, and 

 at birth as 1 to 120. 



The spinal cord does not follow the same progression in growth. In 

 the Calf at mid-term it is only the 421st part of the weight of the foetus, 

 and at birth the -iTOth part. In the Lamb at birth, it is equal to the 

 3-iOth part of the body. 



The heart, which appears so early, is more voluminous in the embryo 

 than towards the termination of gestation and after birth. The heart 

 of a Calf at mid-term equals 125tii of the weight of the body ; that of a 

 Lamb at birth the 120th ; in the adult liuU it is only the 264th part. 



The hnujs have, relative to the total bulk of the fa'tus, a variable 

 weight according to age. In those of the Lamb at fifty-seven days, 

 they are equal to a 20th part of the weight ; in a Calf at mid-term, the 

 33rd; in another at eight months, the 34th ; and at birth, the 35th and 

 55th ; in a Foal a little more than eight and a half months, they were 

 the 33rd ; in Lambs and Goats at birth, they averaged from the 24th 

 to the 61st part of the whole weight. 



The development of the thymus gland also varies considerably accord- 

 ing to age and species. In a Calf at mid-term, its weight was the 137th 

 part of the body; in another at seven months, it was the 197th ; in a 

 Foal at birth, the 200th ; in a Goat at the same period, the 130th ; and 

 in a Lamb eight days after birth, the 22Sth part. It is always propor- 

 tionally more voluminous in Ruminants than Solipeds, and in them and 

 the Pig it extends in front nearly to the larynx. In the Dog it does 

 not get beyond the anterior mediastinum, and is small, though more 

 persistent. It has in rare cases been found in Horses more than three 



