45. 



posterior adductor muscle, figures 33 , aiid 55, pa, does not ap- 

 pear until some liours later. It is always siirrounded by other 

 tissue. The digestive glands, figures 33, 62, and 63, are formed 

 as tvaginations from tiie anterior end of tiie midg^at, vrliicli is nov^ 

 extended posteriorly nearly to the blastopore. The visceral 

 ganglia appear as groups of nuclei, the position of which suggests 

 ectodermal origin. The cerebral poxiches have moved some distance 

 dorsally and posteriorly into the interior of the embryo, and are 

 situated at the end of a single pouch, figures 33 and 55, r, which 

 opens to the exterior between the test cells at the point where 

 the pouches were originally formed. The paired pouches are thus 

 brought to lie in the interior, as diverticula of a single elon- 

 gated pouch. As they are carried into the interior, the cells 

 f03nning their walls divide rapidly, push in, and fill up their 

 cavities, and a commissure is foimed connecting the tv^o masses 

 of colls. The relation of the unpaired pouch to the cerebral 

 ganglia has been 3hO¥/n by tjiree succeeding transverse sections of 

 the same embryo. Figure 53 shows the external opening of the 

 un-paired poiich, figure 59 shows the anterior portion of the cere- 

 bral ganglia, eg, with the laterally compressed unpaired pouch, 



r 

 §, lying beneath them, and figure 60 shows the commissure con- 



necting the ganglia. 



