464 PATTEN AND HAZEN. [Vol. XVI. 



appeared, a solid mass of mesoderm cells is seen at the base 

 of each thoracic appendage. Each mass of cells gradually 

 extends in a lateral direction to form a transverse band quite 

 distinct from those on the opposite side of the body. Beneath 

 the mesoderm, and enclosing the yolk, is a thin non-cellular 

 membrane (PI. XXIV, Figs. 35 and 36), which gradually sep- 

 arates from the central portion of each mesodermic band, but 

 remains continuous with it around its margins. At the same 

 time nuclei appear to migrate from these margins into the 

 membrane. In this way an imperfectly closed sac, the somite, 

 is formed with a very thick outer wall, the somatic, and a thin 

 inner one, the splanchnic layer (PI. XXIV, Figs. 37 and 38). 



Each somite now grows rapidly in a lateral direction, making 

 with its mate on the opposite side almost a half circle. The 

 concave side of the posterior thoracic somites is directed back- 

 wards, that of the anterior somites forwards. 



As the appendages grow in length, a space is formed between 

 the ectoderm, forming the apex of the leg and the thick somatic 

 mesoderm at its base. A few scattered mesoderm cells remain 

 attached to the inner surface of the ectoderm, and they gradu- 

 ally form the walls of a spacious cavity, which later is converted 

 into the blood channel that passes through the center of each 

 leg. We could not certainly determine whether this space is 

 to be regarded as a part of the coelom, as in the abdominal 

 sinuses, or not. 



The further history of the thoracic somites, except the fifth, 

 has not been carefully studied. 



The abdominal somites develop in a different way and are 

 much more clearly defined. They separate from the paired 

 bands of mesoderm formed by the primitive streak as hollow 

 masses, with distinct and continuous walls of nearly uniform 

 thickness. Each abdominal somite is at first quite separate 

 from all the others, and its cavity remains closed for a long 

 time. Figs. 2-8, PI. XXII, are longitudinal sections showing 

 the first three abdominal somites and a part of the primitive 

 streak in an embryo in which the abdominal appendages are 

 beginning to appear. 



In this series the median ends of the somites are thin-walled 



