DEVELOPMENT OF THE FROG'S EGG. 



305 



enteron ventrally placed to them are to be seen in the right half 

 of the sketch. There is no indication at this early stage of a 

 beginning formation of the anlagen either of the liver or of the 

 pancreas. The section represented by n, however, passes 

 through the head region at the level of the eyes. To the left in 

 this sketch there can be identified the cephalic portion of the 

 neural tube with its well-differentiated walls showing a degree of 

 stratification. The neurocele appears clear and contains no cell 



FIG. ii. 



detritus. The neuroblasts do not demonstrate any pathological 

 features. A thin layer of mesenchymal tissue intervenes between 

 the roof of the neurocele and the epidermis. Immediately ventral 

 to this section of the brain cavity the knife has encountered 

 another portion of the normally folded brain stem with its 

 ventricle and, projecting to either side from its ventral aspect, 

 a portion of the optic commissure and stalks proceeding laterally 

 to the optic cups. The cytological appearance of these structures 

 presents no departures from normal. The cavities are free from 

 cell detritus. The lens of the eye on both sides is in its normal 

 position. Stratification of the walls of the optic cup, of the 

 stalk and of the brain vesicle have progressed to their normal 

 relative degree. Immediately ventral to these structures in the 

 median line is shown a section through the pharynx, the cells of 

 which, as well, show none but normal features. The right half 

 of the sketch demonstrates the prominent caudal fin, transected 

 neural tube and notochord, and the caudal extremity of the 

 enteron, the last embedded in the dorsal region of the yolk 



