28O LEONARD P. SAYLES. 



Hypodermis. 



In all the epithelial cells of the hypodermis the nuclei are quite 

 large, all gradations from an oval type (the average measurement 

 of twenty-five of which is 3.3 x 5.8 ju), to the practically spherical 

 one (averaging about 4.2 /*) are to be found. In each a very 

 minute nucleolus, appearing as a mere dot under a magnification 

 of 1,000 X, is visible (Fig. i). Cells of this type form all of the 

 hypodermis from the second segment to the growing region of 

 the tail. At this end, just anterior to the place where the 

 basement membrane is discontinuous due to the formation of the 

 ventral nerve cord, the hypodermal cells are slightly elongated. 

 The nucleoli are slightly larger than in the cells located more 

 anteriorly (Fig. 2). Cells of this type are probably intermediate 

 stages between the active ones of the growing region and the 

 typical hypodermal cells found in the older part of the body. 

 Such an elongation of the cells becomes even more marked where 

 the nerve cord is forming with the result that the hypodermis in 

 the growing tip is two or three times as thick as in the rest of 

 the body wall. This elongation of the cells is most marked 

 ventrally just posterior to the end of the nerve cord. The 

 thickening of the hypodermis includes the dorsal and lateral 

 portions as well as the ventral but it does not extend as far 

 anteriorly on the dorsal side as elsewhere. 



Biilow ('83), in his discussion of the normal growing region, 

 describes this thickening, but makes no mention of any difference 

 in the nucleoli of this region. In posterior regeneration, however, 

 Krecker ('10) describes a marked enlargement of the nuclei and 

 nucleoli of the ventral ectoderm. In the normal growing worm 

 the nuclei are larger near the growing nerve cord and a short 

 distance posterior to it than they are elsewhere (Fig. 2) ; the 

 average size of these nuclei is 3.9 x 7.4 p. The cells in which 

 such a difference is apparent are the ones involved in the forma- 

 tion of the new nerve cord. The nucleoli, on the other hand, 

 are enlarged throughout the entire tip and to a slight degree for 

 a short distance from it. This difference is most decided in 

 the ventral cells, in which the nucleoli may be nearly 2 /x in 

 diameter, but average between i and i^ /JL. 



