12 TUE DEVELOPMENT OF 



give rise to the lon<>- and iiiirrovv Oesophagus. The widen- 

 ing of the stomodeiim to form the buccal cavity is seeii 

 in transverse sections to be in a h)ngitudinal and not in a 

 transverse plane. The Oesophagus goes upwards and for- 

 wards and then bends abmptly backwards to widen out 

 and form the "stomach" {gm). In front of (belovv) this 

 bendthe walls are composed of a single layer of cells, but 

 in the gastric region the epitlielium becomes thickened in 

 places, as can be seen in ligs. 62, 63, 64, 65 and QQ». In 

 fig. 64, which cuts both limbs of the stomodeal invagina- 

 tion near the angle, the gastric portion is seen to have 

 its lumen in the shape of a St. Andrew's cross, the result 

 of a thickening of the epithelium in four places, while at 

 the angles it is but a single cell in thickness. This is ap- 

 parently a provision for the subsequent distention of this 

 region into the large cardiac sac of the adult, a distention 

 not yet possible, on account of the comparatively large 

 amount of yolk not yet metabolized. 



Farther back (figs. 54 and 65), the lumen is mach larger 

 and is vertically compressed, while its walls are thinner. 

 There are, however, to be recognized in this region, three 

 thickenings — one ventral and two lateral — which corre- 

 spond in position and doubtless give rise to the future folds 

 of the pyloric division of the stomach {cf. Mocquard, 

 '83, p. 230). At this stage (excepting that the cuticle and 

 straining hairs are not yet developed) the pyloric portion 

 of the stomach corresponds closely in section with the 

 similar region in the adult amphipod Gammarus. At this 

 stage {cf. fig. 54) the stomodeum certainly opens directly 

 to the yolk, there being not even a mesodermic partition 

 existing between. 



The foregoing account is in close accord with that of 

 Keichenbach ('86) in almost all details except in the 

 communication of the stomodeum with the yolk cavity 



