EMBRYOLOGY OF TADPOLE AND CHICK 



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region and in the head region. A complete sheet of mesoderm now sep- 

 arates ectoderm and entoderm in the embryo from the head region to me 

 primitive streak region, and this sheet separates into two layers, an outer 

 or somatopleure, and an inner known as the splanchnopleure. (Fig. 

 268.) 



The thickened region where the somites form on the dorsal surtace 

 of the embryo along the notochord is called the segmental plate or 

 myotomal region, while the portion extending laterally (which is much 

 thinner) forms the lateral plates. 



The coelom develops between somatopleure and splanchnopleure. 



As the frog's egg has the yolk- 

 mass packed within the embryo, this 

 mass pushes the germ layers close 

 together, so that they are by no 

 means as clearly set apart as in the 

 chick embryo. 



Between the second, third, and 

 fourth somites and the lateral plates, 

 small masses of cells remain closely 

 related to the somatopleure of the 

 lateral plate. It is these cell masses 

 which are to develop into the pro- 

 nephric tubules. All of these struc- 

 tures must be compared with similar 

 developmental structures in the 

 chick embryo, at this point. 



The coelom proper can be seen 

 as a definite space only below the 

 pharynx in front of the liver (Fig. 

 331). The heart will develop in the 

 region where the loosely scattered 

 cells are seen, ventral to the 

 pharynx. 



At 



Fig. 332. 



Diagrams of median sagittal sections of 

 the anterior ends of frog larva?. A. Of a 

 larva just before the opening of the mouth. 

 B. Of a 12 mm. larva (at the appearance of 

 the hind-limb buds), a, Auricle; ao, dorsal 

 aorta ; b, gall bladder ; bh, basihyal cartilage ; 

 eh, cavity of cerebral hemisphere (lateral 

 ventricle) ; e, epithelial plug closing the 

 O3sophagus ; ep, epiphysis ; g, glottis ; h, 

 hypophysis ; H, hind-brain ; hr, cerebral 

 hemisphere; ht, horny "teeth" ; hv, hepatic 

 vein ; i, intestine ; if, infundibulum ; j, lower 

 jaw ; i, liver ; ly, laryngeal chamber ; m, 

 mouth ; M, mid-brain ; mb, oral membrane 



(oral septum) ; n, notochord; o, median por- 

 tion of opercular cavity ; ce, oesophagus ; p, 

 pharynx ; pb, pineal body ; pc, pericardia! 

 cavity; pd, pronephric (mesonephric) duct; 

 pt, pituitary body ; pv, pulmonary vein ; pill, 

 choroid plexus of third ventricle ; pIV, 

 choroid plexus of fourth ventricle ; r, rostral 

 cartilage ; ro, optic recess ; s, stomodaeum ; sv, 

 sinus venosus ; t, thyroid body ; ta, truncus 

 arteriosus ; tp, tuberculum posterius ; v, 

 ventricle; vc, inferior (posterior) vena cava. 



(After Marshall.) 



THE LATER DEVELOPMENT 

 OF THE TADPOLE 



There are details in which the 

 various species of frogs vary, but all 

 pass through the following general 

 method of development. 



It is both interesting and prof- 

 itable to call attention at this point 

 to the fact that, while the frog was 

 one of the earliest forms of animal 

 life studied in the laboratory, and 



