252 THE CHICK. 



They are all three blind pockets, extending somewhat 

 obliquely from the floor of the hinder end of the neural tube 

 into a fused mass of cells just behind the notochord : this mass 

 is really the anterior end of the primitive streak, and therefore 

 corresponds to the anterior lip of the blastopore in the frog 

 (c/. Fig. GO). 



3. The Brain. 



The general history of the development of the brain in the 

 chick is very closely similar to that already described in the frog. 



At the commencement of the second day, and before actual 

 fusion of the neural folds has taken place at any part of their 

 length, the neural canal becomes dilated at its anterior end to 

 form the anterior cerebral vesicle or fore-brain (Fig. Ill, BF), 

 from which the optic vesicles, BO, arise almost at once as lateral 

 outgrowths. Immediately behind the fore-brain, and separated 

 from it by a slight constriction, is a second and rather smaller 

 dilatation, the middle cerebral vesicle or mid-brain, BM. 



The part of the brain behind the mid-brain, about half its 

 entire length, is the hind-brain, BH; this consists of a series 

 of vesicles, separated by slight constrictions, decreasing in size 

 from before backwards, and passing without any limiting 

 boundary into the spinal cord posteriorly. The vesicles of the 

 hind-brain vary considerably in different specimens ; they are 

 usually four or five in number, of which the two anterior ones, 

 at any rate, appear to possess considerable constancy. Their 

 mode of development, and their relations to the nerves and 

 other structures, strongly suggest that they are each equivalent 

 to a single vesicle, such as the mid-brain. 



By the middle of the second day (Figs. Ill and 112) the 

 brain is closed, by fusion of the neural folds, along its entire 

 length ; the point where the folds last meet being at the summit 

 of the fore-brain, in the position subsequently held by the pineal 

 body. 



The walls of the brain are at first of nearly uniform thick- 

 ness in all parts ; and transverse sections of the brain are 

 approximately circular in outline at all parts of its length. 



In the following account the several parts of the brain will 

 be considered in order from behind forwards, and the leading 

 points in their development described. 



