MUSEUM OF COMPARATIVE ZOOLOGY. 91 
While these changes in the cells composing the lamelle have been 
taking place, those which form the ventral wall of the body covering the 
lungs have become definitely 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 cells of the inner layer. The remaining space between these 
two layers forms a part of the body cavity, and like the true lamelle is 
traversed by the blood. The dorsal chitinous covering of the inner layer 
is toothed like the dorsal surface of the true lamelle. 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 with the 
yolk corpuscles. 
In view of the facts learned from sections of this stage this explanation 
seems to me inadequate. Ifthe markings were produced by a centrifu- 
gally directed force the dividing lines would be at least “flush” with the 
surface of the blastema, if not slightly elevated above it. Asa matter 
