T7 



If we examine the embryo on the fourth day, we find 

 the cranial flexure more marked, and the tail fold more pro- 

 minent. 



The cranial nerves appear. The genito-sexual apparatus 

 which had begun on the second day, by the formation of the 

 wolffian duct is now supplemented by the appearance of the 

 wolffian body, which has the structure of the kidney, and is 

 the temporary kidney of the chick, later on the permanent 

 kidney is developed from it, and the excretory duct of the 

 kidney, the ureter is developed from the wolffian duct as a 

 diverticulum. In the same region the genital ridge appears, 

 and this becomes the ovary. By the sixth day, if the chick 

 is going to be a male, the ovary atrophies and disappears, 

 and the testis appears on the outer side of the sexual mass. 

 The wolffian body helps to form part of the testicle, and the 

 wolffian duct becomes its excretory duct, the vas deferens. 

 There is another duct called Miiller's duct, on both sides, it 

 disappears in the male, or at most leaves traces as some 

 suppose in the testis ; in the female it dwindles on the left 

 side with the left ovary and disappears, on the right side it 

 becomes the oviduct and opens into the cloaca. 



The limbs first show themselves on the third day, as 

 ridges on the side of the embryo, at the upper part for the 

 fore limbs, and at the lower part near the tail for the hind 

 limbs, these ridges as it were sprout out, and by the fifth 

 day they have so far developed that the knee and elbow are 

 discernible. I have seen movements of the limbs on the 

 sixth day. During this time the spinal cord has been 

 developing. We saw how the medullary groove closed in, and 

 became the neural canal, the epiblastic cells that were shut 

 in are differentiated into the spinal marrow. The neural 

 canal is arched over by what becomes the bony arch of the 

 vertebrae. 



The mouth is formed by an involution of the face wall. 

 This involution grows backwards till it meets the blind end 

 of the fore gut, the partition disappears, and the mouth thus 

 forms the upper opening of the gut. In the same way an 



