632 HAROLD HEATH, 



somatoblast assumes a character indistinguishable from the bordering- 

 third quartette, and also after the cells posterior to the growth zone 

 become smaller and more irregular in position. Cell cleavages soon 

 reduce the size of these regularly situated cells and it becomes 

 impossible to accurately determine if they pass directly into the evenly 

 arranged cell rows from which the shell arises. In other words it is 

 impossible to state that the cells of the dorsal side are so situated 

 owing to the influence of the future segmentation, a character which 

 appears to be strongly marked in the development of some of the 

 Isopoda (cf. Mc Murrich, '95). 



The first clearly marked indications of the shell occur usually 

 about the fourth or fifth day when a band of somewhat clear cells 

 appears parallel with and a little posterior to the prototroch. Very 

 soon after another appears a short distance behind the first and the 

 process is continued until seven such bands alternate with six narrow 

 rows of darkly staining cells as in Figs. 52, 53. 



LovÉN ('56) described these clear cells filled with granules as 

 the lime secreting cells but a careful study makes this doubtful. 

 When treated with methyl green they stain intensely and this reaction 

 continues as far as I have traced the development of the shell, when 

 these cells come to occupy positions between the valves of the shell. 

 The narrow bands of cells apparently but little difierentiated which 

 alternate with these mucous (?) cells gradually increase in breadth a& 

 a comparison of Figs. 52 and 53 will show and ultimately the cal- 

 careous portions of the shell appear above them. 



The calcareous salts are deposited in the cuticle covering the 

 region of the shell and the presence of the tegmental sense organs 

 (aesthetes) makes it evident that the first part of the shell to form 

 is the tegmentum. The articulamentum appears later but the manner 

 in which it forms is as yet unknown. 



KowALEVSKi ('83) has accurately described the formation of the 

 calcareous portions of the shell and Ischnochiton affords no further 

 important points upon this subject. 



XII. General Considerations. 



1. Cell Homologies among Annelids and Molluscs. 

 Of late years a growing tendency is manifesting itself to look 

 upon the early cleavage stages as something more than a mere mani- 

 festation of simple mechanical forces. Rather are the blastomeres the 



