236 



ZOOLOGY. 



pharyngeal and a posterior intestinal. The growth in length 

 at this stage affects the latter only. Soon little median pits of 

 epiblast appear at the front and hind ends the stomodaeum 

 and proctodaeum (fig. 121) and presently the proctodseum 

 opens into the hind part of the mesenteron, forming the 

 cloacal aperture, anterior and ventral to the blastopore. 

 The stomodaeum remains closed for some time. 



In the side-walls of the pharyngeal region six pairs of 

 vertical thickened ridges appear the visceral arches and 

 the mesoblast-cells in the centre of each begin to secrete a 

 cartilage-matrix. The first of these visceral arches is the 

 mandibular arch, which later becomes bent and divided into 



Fig. 121. MEDIAN LONGITUDINAL SECTION or EMBHTO OF FROO 

 (After Howes.) 



palato-pterygoid and Meckel's cartilage ; the second is tl e 

 hyoid arch, the rest are the four branchial arches. 



The intestinal region is a straight tube, its lateral and 

 ventral walls extremely thick from the amount of yolk 

 they contain. From the front end of this intestinal region 

 a backwardly directed ventral outgrowth is formed, burrow- 

 ing, as it were, into the mass of yolk. This is the rudiment 

 of the liver. 



Behind, the blastopore has closed, the yolk-plug having 

 been absorbed ; and the region of the mesenteron posterior 

 to the proctodsoum, which is continuous with the neural 

 tube, becomes drawn out by the growth of the tail (fig. 121 ). 



