282 EMBRYOLOGY 



are beginning to show. The external nose and the external ear have formed, 

 although the nose is still flattened. The eyes are shifting from the sides to a 

 forward position and are coming closer together. Eyelids have formed. The 

 branchial grooves have flattened out, and a neck, distinct from the head and 

 body, has formed. At this time the limbs have well-formed digits, especially 

 the fingers. The tail is now relatively inconspicuous. 



Internally the telencephalon gives rise to the very large cerebral hemi- 

 spheres, in which there is cellular differentiation. The optic nerve develops 

 by growth of fibers into the optic stalk, and the lens of the eye begins to form 

 lens fibers. Underneath the brain the hypophysis has formed by the union of 

 Rathke's pouch (epidermal part of the hypophysis) with part of the in- 

 fundibulum of the brain. 



The adrenal gland has formed by the migration of the medullary cells 

 from the primordia of the sympathetic ganglia into the cortical region, which 

 is derived from mesoderm. The medulla in some mammals begins to secrete 

 adrenaline very early. The cortex is of especial interest, since it is unusually 

 large in the fetus and is thought to secrete an androgen. The kidney system 

 is continually changing during this period, with the mesonephros completely 

 differentiated and functioning while the metanephros begins to form tubules. 

 The testis and ovary have developed from the primitive gonads, and the 

 secretion of androgens by the testis begins very early, probably at about seven 

 weeks. This early secretion of androgen leads to the formation of' intersexes 

 in cattle twins. When the twins are of opposite sexes and their blood streams 

 intermingle, the androgen from the male co-twin is able to pass through the 

 blood stream into the female co-twin and there stimulate parts of the male 

 sexual apparatus to develop. Such an effect has not been observed for human 

 twins and does not seem at all likely to occur under normal conditions, since 

 there is no intermingling of the blood between the twins. Even in cases 

 where the twins are in the same chorion no intersexuality has been observed. 



Intersexuality of developing chick embryos caused by a change in the 

 concentration of sex hormones is considered in some detail in Chapter 20. 

 Sex reversal can be produced by experimental treatment. 



Eight weeks of development brings the embryonic period to a close and 

 the embryo from now on is termed a fetus. Practically all structures are laid 

 down and the fetal period from eight weeks to birth is characterized by 

 growth, the further differentiation of details of structure, and the onset of 

 function 



