Chap. VI.] GENERAL EMBRYOLOGY. 107 



(37, iv.) is introduced to aid him in overcoming it. It represents 

 the head end of an embryo, the amnion-fold (see p. 108) being 

 omitted to avoid confusion. FB. MB. HB. are the three cerebral 

 vesicles; Ch. the notochord which underlies the second and 

 third. Beneath it lies the blind anterior end of the primi- 

 tive digestive cavity (mes.). The shaded part in front of and 

 beneath it is the mesoblast, which has clef there, just as it cleaves 

 at the sides of the embryo (Fig. 38, iii.), into a somatic layer 

 (So. I.), and a splanchnic layer (Sp. /.), with the body-cavity (b. c.), 

 between them. When it is remembered that the embryo is 

 being folded off at the sides, as well as in the region of the 

 head, the true meaning of the lines So. 1. and Sp. I. in 37, i., 

 will be seen. 



In Fig. 37 X ii., an embryo of about 36 hours, it will be seen 

 that the number of mesoblastic somites has increased, and that 

 the heart (ZT.), has now become obvious. It lies at the point of 

 union of the two vitelline veins (V. V.). The development of 

 this organ is illustrated in Fig. 3 8 A, ii., and is described in 

 Ch. vi. 



In Fig. 37, iii., an embryo of about 48 hours, there is con- 

 siderable change. The mesoblastic somites have increased in 

 number. The heart is more definite, and appears as a coiled 

 tube. Its rhythmic contraction will no doubt be seen in the 

 embryo of this age, and of the age figured in ii. Besides the 

 vitelline veins (V. V.\ the vitelline arteries (V. A.\ are clearly 

 visible, but their connection with the heart is obscured by the 

 mesoblastic somites. In the fore-brain two rounded optic 

 vesicles (op.) are seen to have budded out ; and on either side, 

 in the region of the hind-brain, are the auditory pits (au.) 9 

 from which the auditory capsules will be formed. They are 

 not, like the optic vesicles, parts of the brain, but involutions 

 of the external epiblastic layer. Sp. u. marks the point where 

 the splanchnic layers of the mesoblast of the two sides have 

 united beneath the blind end of the primitive digestive cavity. 

 Anterior to this the splanchnic investment of the mesenteron is 

 complete. Posterior to this the mesenteron has not yet been 

 converted into a tube, and the splanchnic layers diverge. 



