246 ATERS ON THE DEVELOPMENT 



animal. In order to simplify the matter, I will treat each of the following subjects in a 

 separate paragraph. 



1. Alimentary System. 



a. Proctodaeum and Malpighian tubes, b. Mesenteron, pyloric caeca and yolk. 



c. Stomodaeum and salivary glands, d. Corpus adiposum and pigment bodies. 



2. Circulatory System. 



a. Heart and blood corpuscles, b. Lateral and ventral blood sinuses. 



3. Respiratory System. 



a. Embryonic gills, b. Tracheae. 



4. Nervous System. 



a. Ventral cord. b. Brain, c. Suboesophageal ganglion and commissural cords. 



d. Dorsal cord. 



5. Sexual Organs. 



a. Germinal cells, b. Ovaries and Testes. 



6. Germinal Layers. 



a. Ectoderm, its origin and derivatives. b. Mesoderm, its origin and derivatives 

 (splanchnic and somatic layers), and body cavity, c. Endoderm, its origin and de- 

 rivatives. 



The proctodaeum has the shape of a pocket at the time of revolution but it soon elon- 

 gates into a tube ending in an enlargement resembling the cap of a mushroom (pi. 18, 

 fig. 26 ; pi. 19, fig. 1 ; pi. 22, fig. 1 ; Fig. 26). It is composed of two separate layers, an 

 inner epithelial, derived from the ectoderm, and surrounding this a muscular 

 layer derived from the splanchnic layer of the mesoblast. The latter is con- 

 tinuous with the muscular coat of the mesenteron and stomodaeum. When 

 the tube has elongated so that its enlarged end lies within the fourth or 

 fifth segment of the abdomen (counting from behind forwards), there arises 

 near the free end in the median dorsal line a small, trilobed, hollow bud of the 

 ectodermic layer, opening into the lumen of the tube. Each lobe grows 

 rapidly into a small tubular organ, the primitive Malpighian vessel. Each of 

 Fig. 25. these bifurcates at some distance from the proctodaeum, so that there are ulti- 

 8 e?«onof S stomo- mately six of the tubes. The one lying in the median dorsal line grows back- 

 unk) 1 ™ wuii re tue ward for some distance along the proctodaeum ; the two lateral tubes curve in 

 mesenteron. var i us directions through the body cavity. At the time these vessels 



appear the proctodaeum is connected by a dorsal mesentery with the heart. (PI. 23, 

 fio\ 5.) The lumen of the proctodaeum is at first circular in cross section, but as the layers 

 thicken the epithelial lining is thrown into six equal longitudinal folds, which in the 

 anterior part of the tube at this stage almost obliterate the lumen. These folds are par- 

 allel and extend the greater part of the length of the tube. The lumen of the proctodaeum 

 becomes continuous with that of the mesenteron only some time after the muscular layer 

 of these two parts have united. At the time of hatching all traces of the union of the 

 parts have disappeared, but in cross section the mesenteron is still much larger than 

 the proctodaeum. 



