216 



ESSENTIALS OF ZOOLOGY 



,Body Form 



Since the development of the frog has been discussed briefly in 

 the chapter dealing with the bullfrog, it will not be included here. 

 In a developing chicken, which is easily studied, the head fold ap- 

 pears in a fertile egg that has been incubated about twenty hours. 

 The neural groove (primitive spinal cord) begins its formation at 

 about this time. At about twenty-one or twenty-two hours of incuba- 

 tion, there appear on each side of midline and lateral to the neural 



area opaca 

 vitellina 



proamnion 



ectoderm of head 



mesench5nne 



neural fold 



neural groove 

 notochord 



border of fore-gijt 



subcephalic pockct 



margin of ant. 

 intestinal portal 



area pellucida 



.unsegmented 

 mesoderm 



border of mcsodcrtn 





Fig. 64. — Dorsal view of entire chick embryo of about 24 hours of incubation. 

 Notice that the fourth pair of somites is almost developed. (From Patten, Em- 

 bryology of the Chick, P. Blakiston's Son and Company.) 



groove blocklike thickenings in the mesoderm. These are somites, 

 and thej^ are paired opposite each other, marking segmentation in 

 the body. At twenty-four hours of incubation there are four pairs 

 of these somites, and they increase with growth until at thirty-six 

 hours there are fourteen, at sixty hours thirty-two, and approximately 

 forty at four days. Similar somites develop in a somewhat similar 

 way in other vertebrates including mammals. Rabbit, cat, rat, calf, 

 or man is no exception. In a pig embryo 6 millimeters long there 



