104 



ANIMAL BIOLOGY. [Part I. 



end of the young embryo, its anterior end being indicated by 

 the anterior end of the neural canal. 



At this anterior end the neural canal enlarges into three 

 so-called cerebral vesicles, the precursors of the fore-brain, mid- 

 brain, and hind-brain. They are seen in Figs. 36 and 37, FB., 

 MB., HB. 



The folding off of the Embryo. We have seen that in the 

 rabbit and the fowl the embryo, marked out by the neural 

 tube, the underlying notochord, and the mesoblastic plates, 

 occupies but a small area of the blastoderm. In front of the 

 neural tube a crescentic groove (Fig. 35, iii.) makes its appear- 

 ance, bending round the anterior end of the embryo, with the 

 horns of the crescent pointing backwards. This marks off the 

 head-fold. Behind the neural tube a similar groove is subse- 

 quently formed, with the crescentic horns directed forward, 

 giving rise to the tail-fold, and soon the horns of the crescent 

 become connected at the sides by lateral folds. All these 

 grooves gradually become constricted beneath the embryo. 



Take your knife or pencil-case and lay it upon the table. 

 Spread over it your handkerchief, and tuck the handkerchief 

 under the ends. Pass a piece of string round the ends in the 

 tucks, single knot it, and gradually draw it tight. The string 

 will form curved grooves under the ends, and straight lateral 

 grooves along the sides ; and as you pull it tight the grooves at 

 the ends become deeper and deeper, until the part of the hand- 

 kerchief containing the knife is folded off from the rest, and 

 only connected by a puckered stalk. This will illustrate in a 

 clumsy but comprehensible fashion the beautiful process by which 

 the embryo is folded off from the rest of the blastoderm, which 

 in the case of the fowl envelopes the whole of the yolk, and is 

 called the yolk-sac (Fig. 38, i. y. s.), and in the case of the rabbit 

 contains an albuminous fluid, and is called the umbilical vesicle. 



It must be remembered that in both these types the hypo- 

 blast covers a cavity the primitive alimentary cavity. During 

 the process of the folding off of the embryo this cavity is not 

 obliterated, but the walls of the embryo folding in beneath it, 



