DEVELOPMENT. 193 



oped a sort of secondary cleavage of the parts, which by the rapid 

 growth of the parts included between 

 the clefts resembles a budding (Fig. FIG. 64. 



64). It is by the growth of these 

 buds or processes that the outline of 

 the face is formed. From each side 

 sprouts the superior maxillary pro- 

 cess, and the processes unite in the me- 

 dian line, meeting the nasal or inter- 

 maxillary process from the upper bor- 

 der of the cleft. The portion below is 

 cut off by a branchial cleft, which, be- 

 coming the mandlbular process, forms Development during First Month. 



the lower jaw. 



What defects in the face are due to faulty development of these 

 processes ? 



When the processes do not unite as they should, various defects 

 occur ; most common are those about the mouth, cleft palate and 

 hare-lip, by failure of the superior maxillary processes to unite or 

 by failure of the intermaxillary process to unite with the max- 

 illary. 



Do the other branchial clefts persist in later life ? 



No. They become closed as they accomplish their use in devel- 

 oping certain organs : as pathological factors, however, we are often 

 convinced of their non-union or of flaws in their development, cysts 

 and tumors of various kinds and certain fistulse being attributable 

 to this cause. 



How are the extremities developed? 



They develop as buds from the somatoblast early in foetal life, 

 and the formation of the joints and lesser details of structure are 

 gradually worked out. At about the third month the separation 

 of the fingers and division of the extremity into joints is about 

 completed. The arm develops somewhat in advance of the leg, and 

 grows rather more rapidly in the earlier period of intra-uterine life. 



What two forms of circulation are found in the embryo ? 



The earliest is the viteUine circulation, in which vessels from the 

 foetus pass over the yelk-sac and carry nutrition to the growing 

 organism. There is formed a placental circulation, in which the 

 maternal blood furnishes the elements of food. 

 13 Phy. 



