MUSEUM OF COMPARATIVE ZOOLOGY. 91 



While these changes in the cells composing the lamellae have been 

 taking place, those which form the ventral wall of the body covering tlie 

 lungs have become detinitely arranged in two layers — an outer one, 

 which is distinctly continuous with the hypodermis of the surrounding 

 regions of the body, and like it is composed of a single layer of close-set 

 pigmented cells, with large elongated nuclei, and a deeper one composed of 

 elongated, unpigmented cells, with smaller nuclei. These two layers are 

 continuous with each other at the anterior lip of the pulmonary opening. 



The two layers of cells are connected by cellular strands, probably of a 

 muscular nature, which run obliquely downwards and forwards from the 

 inner to the outer layer. They appear to be simple elongations of the 

 body of the ceUs of the inner layer. The remaining space between these 

 two layers forms a part of the body cavity, and like the true lamellae is 

 traversed by the blood. The darsal chitinous covering of the inner layer 

 is toothed like the dorsal surface of the true lamellae. According to the 

 figures of the adult structure given by MacLeod ('84, PL I. fig. 3), one 

 would expect to find the chitinous layer of this surface plain rather than 

 complicated. 



I have not yet succeeded in demonstrating the existence of any mus- 

 cular differentiations in the 2-cell columns, such as is described and 

 figured by MacLeod for the adult. 



IV. — General Considerations relative to some of the 



Phenomena. 



The discussion concerning the nature of the peripheral layer of proto- 

 plasm — or blastema — in the early condition of the laid egg, has al- 

 ready been referred to. Sabatier ('81) is the only author who has 

 attempted to assign a cause for the division of this layer into definite 

 polygonal areas. He concluded that the cause resides in the movements 

 of the internal protoplasm which migrates towards the periphery. This 

 hyaline protoplasm gushing forth from between the yolk corpuscles at the 

 surface produces, in his opinion, certain lines of division in the blastema. 

 The areas thus formed naturally correspond in size and position \n.ih. tlie 

 yolk corpuscles. 



In view of the facts learned from sections of this stage this explanation 

 seems to me inadequate. If the markings were produced by a centrifu- 

 gally directed force the dividing lines wo\ild be at least " flush " with the 

 surface of the blastema, if not slightly elevated above it. As a matter 



