150 



DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



. Diocoel 

 -Diencephalon 

 Infundibulum 

 'Anterior cardinal vein 

 Pigmented layer of eye 

 'Optic cup 



Anterior tip of notochord 



Rhombocoel 



Rhombencephalon 



Anterior cardinal vein 



Notochord 



Cranial nerves VII 

 ond VIII 



External gill 



The 7 mm. frog tadpole: transverse sections. {Top) Through the level of the 

 thyroid gland. {Bottom) Through the level of the heart. 



the frog, but persists in the embryos of some vertebrates. At about 

 the level of the second to the fourth somites, the center of each nephro- 

 tome becomes evacuated to develop a nephrocoel. This is the very 

 beginning of the embryonic head kidney or pronephros. The effect of 

 the expansion of the intermediate mass of mesoderm, due to the de- 

 velopment of the nephrocoel, was seen on the surface of the earlier 

 embryo, lying just dorsal and posterior to the gill plates. 



Origin of the Mesodermal Epithelium, Coelom, and Its 

 Derivatives. In the more ventrally placed hypomere (lateral plate 

 mesoderm) we find a continuous split which separates the mesoderm 

 into an outer, parietal or somatic layer and an inner, visceral or 

 splanchnic layer of mesoderm. The outer somatic mesoderm, in con- 

 junction with the adjacent body ectoderm, is called the somatopleure, 

 and gives rise to the skin with its blood and connective tissue. The 

 inner splanchnic mesoderm, in conjunction with the gut endoderm. 



