386 TH. H. MONTGOMERY jr., 



1. The undifferentiated hypodermal Cells. 



Under this heading are included all hypodermal elements not in- 

 cluded under the other captions ; in all respects these cells appear the 

 most primitive. 



These cells constitute a single-layered, rather flattened epithelium 

 (Fig. 6) in which cell boundaries could not be determined; this epi- 

 thelium bears a continuous chitinous cuticle of greater thickness than 

 the epithelium, the surface of the cuticle having short spines. 



The nuclei (Figs. 1, 6) appear round on surface view, oval on 

 lateral view, and have delicate membranes. To each of them usually 

 one, sometimes two or three, true nucleoli are present ; these are not 

 vacuolated, show the usual staining reactions , and have a more or 

 less central position. There is no chromatin nucleolus. The chromatin 

 appears in the form of small, irregular granules, occasionally in a 

 reticular arrangement; it stains less densely than that of any other 

 cells of the body. 



3. The hypodermal Cells of the Extremities. 



The cells of that portion of the hypodermis which covers the 

 extremities (feet) are morphologically modified, this modification un- 

 doubtedly standing in close physiological correlation with the production 

 of the large curved, cuticular hooks of the ventral (distal) surface of 

 the feet. At the proximal margin of a foot, these cells are least 

 modified, and most modified at the distal edge. From proximal to 

 distal margin a perfectly graduated series of transitional stages is 

 found. To study this series of changes, that is, the complete line of 

 development, one need only study a cross section of the body in a 

 plane where a pair of extremities lie, and compare the cells in con- 

 secutive order. 



The hypodermal cells of the body wall, at the point where the 

 latter connects with a foot, are undifferentiated and like those just 

 described. The cells at the proximal edge of a foot, however, show 

 the gradual appearance and growth of a chromatin nucleolus, at first 

 no larger than the other chromatin granules, but recognizable by its 

 deep staining intensity with haematoxylin and methyl green. The 

 nuclei are somewhat larger than those of the undifferentiated hypo- 

 dermal cells, and their true nucleoli frequently number two or three. 

 In most cases the chromatin nucleolus, when it may first be distin- 

 guished, appears in contact with one of these true nucleoli, though 



