248 AYERS ON THE DEVELOPMENT 



tact with the proctodaeum the walls of the mesenteron are thrown into folds. At present 

 I am unable to say whether these folds go to form diverticula of the mid gut or disappear 

 altogether. The large yolk cells which, as has been stated, appose themselves to the ger- 

 minal band to form the mesodermic elements are not all employed to form the middle 

 germinal layer, but some of them become arranged as a superposed layer, — the endoderm. 

 At an early stage one cannot distinguish between these cells, whether they are to form 

 mesodermic or endoclermic elements. Previous to the time of revolution the mesenteron 

 is formed slowly, but after this act it rapidly becomes a definite sac enclosing the yolk 

 mass. The anterior end of the mesenteron is in connection with the serosa sac, and since 

 the latter passes bodily into the embryo, its cells grade so insensibly into those of the walls 

 of the mesenteron that it becomes difficult to distinguish the place where one begins and 

 the other ends. After revolution the yolk passes from the yolk sac into the mesenteric 

 cavity through a circular opening in the body wall (i. e. amnion) back of the head. (PL 

 19, figs. 1, 2 ; pi. 22, fig. 1.) It is forced into the body of the embryo by the contraction 

 of the walls of the yolk sac. As a consequence of this continued thickening and contrac- 

 tion, many of the nuclei of the serosa cells are set free from their cells and pass with 

 the yolk into the body through the circular pore back of the head. After passing this 

 point some of them find their way at either side of the oesophagus into the body cavity, 

 while others go directly into the open end of the heart; the majority of them, however, 

 pass with the yolk into the enteric cavity and aid in the assimilation of this mass. The 

 remnant of the yolk sac is seen for a time as a plug-like projection from the median dorsal 

 wall behind the head. It ultimately passes into the body cavity and is absorbed. 

 When within the embryo there is a faint lumen between the apposed walls of the yolk- 

 sac, which is continuous with the cavity of the mesenteron. (PL 19, figs. 4, 5 ; pi. 22, 

 fig. 1 ; pi. 23, fig. 2.) Up to the time of hatching the mesenteron is more or less distended 

 with yolk matter and its walls are in consequence very thin, but as the yolk is assimi- 

 lated the walls become thicker and the diameter of the tube is much diminished. 



Owing to the rapid growth of the embryo after revolution, the yolk mass is quickly taken 

 within the body walls. The greater part of it is consumed before the walls are closed. 

 The assimilative function is so active that the amount of food prepared exceeds the amount 

 that can be made use of by the growing tissues, and in consequence this surplus is stored 

 up in two dorso-lateral fat bodies (pi. 19, fig. 1), which lie on either side of the heart and 

 extend from just back of the head to the tip of the abdomen. These bodies are yellow, reach 

 their greatest development soon after the closure of the dorsal wall and have entirely dis- 

 appeared at the time of hatching. In pi. 19, fig. 12, is shown one lobe of the corpus adi- 

 posum of the left side. It is an irregularly shaped lobe composed of loosely connected 

 grayish corpuscules. The lobes, one to each segment, project outward from the longitudi- 

 nal connecting body. Within the grayish mass are numerous yellow fat-drops and an 

 irregular network of black pigment. The muscular wall of the mid gut is formed by the 

 splanchnic mesoblast before revolution. About the time of revolution, however, there is 

 deposited upon this layer another, which at first is very thin and irregular. After the 

 dorsal wall of the abdomen is closed over, the mesoderm is quite well formed in the region 

 of the proctodaeum, (PI. 22, figs. 1, 9.) The cell walls are very indistinct (if present at 

 all) and cannot be made out on specimens treated with osmic-acetic acid solution. The 

 nuclei vary in size but are usually somewhat larger than the nuclei of the surrounding 



