HISTOLOGY AND DEVELOPMENT OF MYRIOTHELA PHRYGIA. 529 



protoplasm. Such cells form a very striking feature of osmic 

 acid preparations. The fluctuations in the size of the individual 

 cells in the endoderm of the blastostyles naturally leads to a 

 corresponding fluctuation in the total bulk of that tissue. In 

 specimens taken in May or June it sometimes almost fills the 

 cavity of the blastostyles. 



We therefore have in the blastostylar endoderm, in addition 

 to the scattered gland-cells^ the following : — (1) Small dense 

 cells with nucleus and nucleolus which stain deeply. (2) Cells 

 whose protoplasm is completely occupied by vacuoles, each of 

 which, some of which, or none of which contain nutritive 

 spheres. And there are numerous intermediate stages between 

 (1) and (2), with either a small or a large portion of the cell sub- 

 stance occupied by vacuoles. The protoplasm of cells (1) and 

 (2), or of the intermediate stages, appears in osmic acid prepara- 

 tions to be remarkably dense and almost glassy, and the vacuo- 

 lation is limited to the large vacuoles which embed the nutri- 

 tive spheres. But other vacuolate cells occur, forming a third 

 class, whose protoplasm is not of this character, but is so 

 occupied by vacuoles of all sizes as to give the whole cell a 

 very characteristic appearance (fig. 22). The vacuoles always 

 differ in size in diff'erent parts of the cell, being larger near the 

 free surface where they may contain young nutritive spheres. 

 These cells, forming the third type of vacuolate cell to be seen 

 in sections through the blastostyle, may be regarded as cells 

 which are actively forming nutritive spheres. 



Before considering the processes going on in the vacuolate 

 cells generally, it will be well to turn to the nutritive spheres 

 themselves. The endoderm generally contains three kinds of 

 formed bodies — that is to say, bodies which are not of the 

 nature of material merely ingested by the cells, but rather are 

 new formations resulting from the activity of those cells. 



Ofthese the most common are small spherical bodies, gene- 

 rally about 3 ju in diameter. These crowd the endoderm- 

 cells in great numbers, but are always most numerous in the 

 foot and blastostyles. They stain uniformly, but not so deeply 

 as to become opaque, with osmic acid, and when perfect appear 

 VOL. XXXII, PART IV. NEW SER. N N 



