Chap. VI.] 



GENERAL EMBRYOLOGY. 



103 



(Fig. 35, iii.). Thus the primitive streak occupies the same 

 position as the blastopore of the frog, and is very probably 

 homologous with that structure. On the dorsal side of the 

 blastopore of the frog hypoblast and mesoblast come into 

 close relation with the inflected epiblast. From the primitive 



MB. 



no. 



bl: 



FIG. 36. THE NEURAL TUBE. 



i. and ii. Dorsal views of frog embryo, iii. Longitudinal section of embryo 

 in about the same stage as ii. 



&?. Blastopore. ch. Notocliord. FB. Fore-brain. HB. Hind-brain. 

 liy. Hypoblast. MB. Mid-brain, nc. Neural canal, nf. Neural fold. 

 me. Mesoblast. n. g. Neural groove. n. e. c. Neurenteric canal. 

 yk. c. Yolk-containing cells. 



streak of the fowl and rabbit the mesoblast grows inwards 

 (Fig. 35, iv.), and at the anterior end of the primitive 

 streak this mesoblast meets that produced in the neural 

 region from the hypoblast (Fig. 35, v.), thus effecting at this 

 point an indirect junction of epiblast and hypoblast. In 

 some of the lower vertebrate types, moreover, among those 

 which possess a primitive streak, a passage here perforates the 

 blastoderm, and leads into the primitive digestive cavity. So 

 that it is probable that the primitive streak is homologous with 

 at least the dorsal part of the blastopore. In the fowl, the point 

 where the spreading blastoderm finally coalesces, so as to enclose 

 the yolk (p. 100), possibly marks the ventral portion of the blasto- 

 pore. What relation, if any, the pseudo-blastopore of the 

 rabbit's ovum bears to the true blastopore is riot known. In 

 any case, the blastopore of the frog, and the anterior end of the 

 primitive streak of the fowl and the rabbit, mark the posterior 



