98 FORMATION OF SHELLS OF ANIMALS, ETC., 
combination is the blastema or nutritive fluid of physio- 
logists advocating the cell-germ theory; and, containing 
all the elements of the structures in progress of formation, 
it may be considered also as the nutritive fluid, according 
to the view now advocated. There is, however, this 
difference in the explanation of the process by which this 
fluid is elaborated. According to one view the vital prin- 
ciple is imparted to separate portions of matter, endowing 
them with distinct and individual powers, to be employed 
in accordance with certain hypothetical laws ; according to 
the other, vitality is considered to disdain all such sub- 
division of a general principle and such co-operations, yet 
still to produce the same effects on this fluid but by the 
employment of certain physical forces from whose operation 
matter cannot be dissociated. I shall next consider the 
structure and mode of formation of shell-tissue as it occurs 
in the shells of molluscs. Of these I shall select such as 
are the most common, and in which the facts observed 
connected with their structure and development admit of 
bemg most easily verified. The shell of the mussel, and 
still more that of the oyster, furnishing all that is necessary 
to be observed in this class of shell-tissues, I shall confine 
my observations to them, taking it for granted that all the 
rest of this class of animals have their shells formed upon 
the same plan. The shells of the mussel and oyster are 
laminated like that of the crab or lobster, and grow in the 
same manner by the successive additions of new layers to 
the internal surface of the layer last formed; so that each 
valve of a shell presents the form of a cone, whose summit 
is the valve or layer first formed, and whose base is the 
layer formed last,—the layer in immediate contact with 
the surface of the mollusc. Each layer also consists of a 
membrane calcified, especially on its external surface 
