200 THE FOURTH DAY. [CHAP. 



epithelium, d. alimentary canal. M. commencing me- 

 sentery. S.O. somatopleure. S.P. splanchnopleure. V. 

 blood-vessels, pp. pleuroperitoneal cavity. 



velopments of the ridge, the rest of which becomes less 

 and less prominent as they increase in size. Each bud, 

 roughly triangular in section, consists of somewhat 

 dense mesoblast covered by epiblast which on the sum- 

 mit is thickened into a sort of cap. The front limbs or 

 wings (Fig. 67) arise just behind the level of the heart, 

 and the hind limbs in the immediate vicinity of the 

 tail. The first traces of them can be seen towards the 

 end of the third, but they do not become conspicuous 

 till the fourth day, by the end of which the two pairs 

 may be already distinguished by their different shapes. 

 The front limbs are the narrowest and longest, the hind 

 limbs being comparatively short and broad. Both are 

 flattened from above downwards and become more so as 

 their growth continues. 



In the head, the vesicles of the cerebral hemispheres 

 are rapidly increasing in size, their growth being enor- 

 mous as compared with that of the thalamencephalon or 

 vesicle of the third ventricle. The mid-brain is now, as 

 compared to the other parts of the brain, larger than at 

 any other epoch, and an indistinct median furrow on its 

 upper surface indicates its division into two lateral 

 halves. The great increase of the mesoblastic contents 

 of the secondary optic vesicle or involuted retinal cup 

 causes the two eyeballs to project largely from the sides 

 of the head (Fig. 69, Op). The mass of mesoblast which 

 invests all the various parts of the brain, is not only 

 growing rapidly below and at the sides, but is also 

 undergoing developments which result in the formation 



