78 THE SECOND DAY. [CHAP. 



first (Fig. 21, mb, Jib). They may be also called the "fore 

 brain," the " mid brain," and the " hind brain," for into these 

 parts will they eventually be developed. 



14. The optic vesicles, lying underneath the epiblast, 

 towards the end of the day are turned back and pressed 

 somewhat backwards and downwards against the sides of 

 the first cerebral vesicle or fore brain, an elongation of 

 their stalks permitting this movement to take place. The 

 whole head becomes in consequence somewhat thicker and 

 rounder. 



15. Before the end of the day the fore brain, by a pro- 

 cess similar to that whereby the optic vesicles were formed, 

 viz. undue growth followed by constriction, has begun to 

 bud off two small vesicles in front ; these are the vesicles 

 of the cerebral hemispheres, which subsequently become the 

 most conspicuous part of the brain, but up to the end of the 

 day are still very small and inconspicuous. 



16. The notochord, whose origin was described in the 

 account of the first day (Chap. in. § 5), is during the whole of 

 the second day a very conspicuous object. It is seen as a trans- 

 parent rod, somewhat elliptical in section (see Fig. 20, ch), 

 lying immediately underneath the medullary canal for the 

 greater part of its length, and reaching forward in front as 

 far as below the centre of the second cerebral vesicle, where 

 it ends either in a point (Remak), or in a rounded knob 

 (Baer, Dursy, Entwickelungsgeschichte des Kopfes). The ex- 

 act relations of its termination will be discussed later on. 



Round the anterior termination of the notochord, the 

 medullary canal, which up to the present time has remained 

 perfectly straight, towards the end of the day begins to curve. 

 The front portion of the canal, i,e. the fore-brain with its 

 optic and cerebral vesicles, becomes slightly bent downwards, 

 so as to form a rounded obtuse angle with the rest of the 

 embryo. This is the commencement of the so-called cranial 

 flexure. 



17. Lastly, as far as the head is concerned, the rudiment 

 of the ear appears about this time on the dorsal surface as 

 a small depression or pitting of the epiblast on either side of 

 the hind-brain (Fig. 21, au. p). 



18. We left the heart as a fusiform body slightly bent to 

 the right, attached to the under wall of the foregut by the 



