io6 



ANIMAL BIOLOGY. 



[Part I. 



in the middle line, is seen the first cerebral vesicle or fore- 

 brain (FB.\ followed by the mid-brain (MB.), and the hind- 

 brain (HB.\ the latter passing into the myelon, still unclosed 

 behind. The line marked a is the amnion fold (see p. 108). 

 Pr. g. marks the remains of the primitive groove. The noto- 

 chord (ch.) is seen through the neural tube. On either side 



Sol. U. Sju. 



FIG. 37. CHICK EMBRYOS. 



i., ii., Hi. Chicks of 24, 36, and 48 hours incubation, iv. Diagrammatic 

 section through anterior end of i. 



a. Amnion fold au Auditory vesicle, b. c. Body-cavity (ccelom). Ch. 

 Notochord. FB, Fore-brain. H. Heart. HB. Hind-brain. MB. Mid- 

 brain, MS. Mesoblastic somites. nies. Mesenteron. op. Optic vesicle. 

 Pr. g. Primitive groove. So. I. Somatic layer. Sp. I. Splanchnic layer. 

 Sp. u. Union of splanchnic layers. V. A. Vitelline artery. V. V. Vitelline 

 vein. 



of the spinal cord (myelon)) are seen the cubical masses (meso- 

 llastic somites), into which the vertebral plate is segmenting. The 

 lines So. I. and Sp. I. mark the limits of the somatic and splanchnic 

 layers of the head-fold. 



As this may present a difficulty to the student the diagram 



