606 



DEVELOPMENT 



[CHAP. XLII. 



the chick betwen the sixth and seventh days as a small spherical 

 mass of blastema on each side of the root of the neck. In struc- 

 ture the thyroid resembles that of the spleen. Professor Goodsir 

 describes the thymus, thyroid, and supra-renal capsules as arising 

 from the membrana intermedia ; but Mr. Gray, on the other hand, 

 has pointed out that they are formed from separate and indepen- 

 dent masses of blastema. 



The development of the thymus gland, and its subservience to 

 respiration, have been already considered in p. 521. 



Development of the Alimentary Canal. The alimentary canal is 

 first seen in the form of an elongated straight tube, in which oil 

 globules may be distinguished. According to Reichert's observa- 

 tions on the embryo of the frog, the walls of the intestine appear 



Fig. 28G. 



Embryo dog, showing the junction of the umbilical vesicle, with the intestinal canal, 

 a. Nostrils. 6. Eyes. c. First visceral arch. d. Second visceral arch. e.f. Right and left 

 auricular appendage, g. h. Right and left ventricle of the heart, i. Aorta, k. Liver, 

 between the two lobes of which is represented the divided omphalo-mesenteric vein. I, 

 Stomach, m. Intestine, communicating with the umbilical vesicle by the omphalo-mesenteric 

 duct. n. Umbilical vesicle, o. Wolfflan bodies, p. Allantois. q. The upper extremities. 

 r. The lower extremities. After Bischoff. 





