CHANGES IN THE OVUM. 109 



the Sheep, towards the end of the first quarter of gestation, the mem- 

 branes alone weigh from five to nine times more than the fcetus ; 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 foetus, with a relative diminution in the proportion 

 of the fluids ; until about the end of the fourth month they only repre- 

 sent something like a third of the weight of the fcetus. 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 foetal lamb 

 at the fortieth day has scarcely acquired the 300th part of the weight it 

 has at birth ; at two months it has only a 60th, but now it grows more 

 rapidly ; so that at the expiration of the fourth month, the foetus has 

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



The inequality of the growth of the different organs is also very no- 

 table ; 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 fcetus of the lamb at the fifty-seventh day, the proportion 

 of the entire brain to that of the body in weight is as i to 15 ; at the 

 seventieth day it is as i to 55 ; and at birth as i to 65. At mid-term 

 with the calf, its weight is to that of the body as i to 53, and at birth as 

 I 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 470th part. In the lamb at birth, it is equal to the 340th 

 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 125th of the weight of the body ; that of a lamb 

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



The lungs have, relative to the total bulk of the foetus, a variable weight 

 according to age. In those of the Sheep at fifty- seven days, they are 

 equal to a 20th part of the weight ; in a calf at mid-term, the 33d ; 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 33d ; in 

 lambs and goats at birth, they averaged from the 24th to the 6ist part of 

 the w^hole 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 228th part. It is always proportion- 

 ally 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 years old, but 

 as a rule in the adult animal it is only represented by a small and vari- 

 able quantity of adipose tissue. 



The liver is extremely voluminous in the early period of the embryonic 

 and foetal life, but decreases proportionately as gestation approaches its 



