QUESTIONS AND ANSWERS 275 



with the fetal bladder by means of the uraehus. The structure of the 

 allantois is slightly fibrous with an epithelial layer. 



Describe the development of the skin of the foetus. 



The skin is developed from the middle and outer layers of the 

 blastoderm (the mesoblast and epiblast). Prom the mesoblast, the 

 skin-proper, or derma, is formed, and from the epiblast, the epi- 

 dermis and epidermic tissues, such as the nails, hair, sudoriparous 

 and sebaceous glands are formed. Blood-vessels are apparent in 

 the derma as early as the third month. The horny and mucous 

 layers of the epidermis are distinguished soon after. As the foetus 

 increases in volume, the epidermis exfoliates and its debris is found 

 in the liquor amnii. Hair is developed in the epidermic laminae 

 which are prolonged into the substance of the derma. It is seen on 

 the eyebrows, lips and the joints of the limbs early in fetal life, 

 and by the sixth or seventh month covers the body. 



Trace the development of the nervous system. 



The first trace of the central nervous system, the neuraxis, is 

 found in the primitive groove, resulting from an invagination of the 

 thickened ectoderm. The groove deepens and the neural folds which 

 border it increase in prominence until they fuse together above, 

 making a closed canal, lined by ciliated epiblastic cells. This canal 

 persists throughout the animal's life as the central canal in the 

 spinal cord, and as the ventricles of the brain. The deeper epiblastic 

 cells in the walls of the canal develop into the nerve-cells of the 

 cerebrospinal axis. The anterior part of the canal shows three 

 successive dilatations which ultimately become the various divisions 

 of the brain. The spinal motor nerves arise from the cord and 

 the sensory branches appear to be developed from the spinal ganglia 

 which are separately formed. 



Describe the situation of the pregnant uterus in the mare and cow. 



The pregnant uterus occupies the floor of the abdominal cavity. 

 In the mare, it is displaced somewhat to the left by the pelvic 

 flexure of the colon and passes beneath the latter to the left of the 

 median line until it reaches the diaphragm. In the cow, the rumen 

 displaces the pregnant uterus, causing it to pass downward and 

 slightly to the right of the median line to reach the diaphragm. 

 Describe the position of the foetus in the ninth or tenth month of preg- 

 nancy. 

 The head is directed backward and ventrally in such a manner 

 that the lower jaw is in contact with the throat, and the mouth 

 touches the sternal region. The neck lies against the maternal 

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