148 



DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



somites show the greatest differentiation. About four pairs will be 

 seen in the 2,5 mm. embryo. The somites sever their connections with 

 the intermediate cell mass and the more ventral lateral plate meso- 

 derm. They become blocks of cells within each of which there de- 



(Mandibulor) Visceral arch I 



(Hyoid) Visceral orch II 



Visceral arch II 



External gill I 



Visceral arch IV 



External gill II 



Viscerol arch V 



Oesophagus 



Midgut 



Hindgut 



Stomodeal cleft 



Olfactory pit 

 Olfactory placode 

 Hypophysis 

 Internal carotid artery 



Aortic arches 



Pharynx 



Pronephric tubules 

 Splanchnic mesoderni 



Somatic mesoderm 



Yolk endoderm 



Pronephric duct 



Frontal (horizontal) section of the 7 mm. 

 frog larva at the level of the pharynx. ( Redrawn 

 and modified from Huettner.) 



velops a cavity, the myocoel. This myocoel is eccentric in position, 

 appearing displaced toward the lateral margin of the somite. As a 

 result of this, the outer layer of somite cells is the thinner. It is known 

 as the dermatome, or cutis plate, having to do with the derivatives 

 of the dermis and of the appendage musculature. The inner layer of 



