590 WILLIAM PATTEN. 



The mesoderm separates early along the median line, forming 

 a pair of lateral bands, which soon break up into segments, in 

 each of which arises an imperfectly closed space, the body 

 cavity. The splanchnic mesoderm grows under the yolk in 

 alternating, continuous, and interrupted bands. Through the 

 openings in the interrupted bands some of the yolk-cells pass 

 into the body cavity. Before the embryonic membranes rup- 

 ture the yolk- cells have already begun to form the lining of 

 the posterior part of the mesenteron. 



Tracheae are formed in all the post-oral segments, except the 

 last two or three segments of the abdomen. 



The spinning and salivary glands are formed by special ecto- 

 dermic invagination on the inner side of the second maxillae and 

 the mandibles respectively. 



The Malpighian vessels arise as six separate evaginations of 

 the blind end of the proctodaeum. 



The embryonic membranes, after rupturing, retreat towards 

 the back, where they form the dorsal organ, which at one stage 

 is simply a vertical tube-like invagination that quickly disap- 

 pears, and the whole mass of cells which constitute it gradually 

 sinks into the yolk and is absorbed. 



A solid cord of cells is formed in the median dorsal region by 

 the fusion of the edges of the mesodermic folds. The cord 

 becomes hollow and forms the heart. 



Concluding Remarks. 



No group in the animal kingdom shows such a wonderful 

 morphological and physiological diversity as the Tracheata, and 

 this tendency to vary is as strikingly manifested in the develop- 

 ment of the individual as in the adults themselves. Even 

 closely related groups present the widest variations in their 

 fundamental processes of development. Hence the few facts 

 obtained from the development of one species do not warrant 

 wide generalizations. 



The variation in the amount of yolk always aflFords a con- 

 venient and often a very plausible explanation for these modified 

 processes of development, but it will not account for everything. 



