256 AYERS ON THE DEVELOPMENT 



ner of their formation closely resemble the ventral appendages, except that they are 

 retsricted in their growth by the cuticula and only increase in size at each ecdysis. The 

 ectoderm of the insect has now lost its cellular character and constitutes a syncytial layer 

 in close contact with the inner surface of the cuticula. 



The serosal membrane of Oecanthus affords excellent material for studying the structure 

 of the cell and the changes which its different parts undergo during the process of division. 

 In Fig. 17 is represented a portion of the fresh serosa, treated with dilute acetic acid. The 

 reagent is just beginning to affect the cells. Adjacent cells are joined together by intra- 

 cellular matter which is to be considered as belonging to the cell substance. Near the cen- 

 tre of each cell is seen the usually spherical nucleus. Before the reagent had affected 

 them, these nuclear bodies appeared to be filled with a finely granular protoplasm, — often 

 showing a bipolar arrangement, — which contained from one to three highly refractive nucle- 

 oli. Sometimes a small area was to be distinguished about one, or each, of the nucleoli. 

 The boundary of the nucleus, although sharply defined against the cell protoplasm, seemed 

 to fade insensibly into the nuclear substance. After the action of weak acid the membrane 

 showed a sharp double contour and the nuclear substance became more coarsely granular. 

 Some of the nucleolar bodies are seen to be centres from which the nuclear substance 

 radiates either in the form of distinct fibres or as rows of granules. Many of these rays 

 are finally deflected toward, and centre in, the opposite pole. In nuclei in which two or 

 three of these bodies are present, there are frequently seen two centres of radiation 

 between which lies the third nucleolar mass. This third body corresponds, in its rela- 

 tive position to the centres of attraction, with the so-called Zellplatte in the process of 

 cell division. (Fig. 17 ; pi. 21, figs. 15, 16, 17, 20. 22.) In other nuclei there is seen a 

 distinct spindle structure, at either end of which are placed the nucleoli which thus form the 

 centres of radiation. (PL 20, figs. 32, 33.) The stages in this process of nuclear division 

 are evidently very similar to those which have recently been described for ce/^-division by 

 numerous writers. 1 One sometimes finds within a single cell wall two nuclei lying in con- 

 tact (pi. 20, fig. 31), — each of which contains a nucleolus with radiating filaments similar to 

 those of the single nucleus, — and in such mutual relationship as to indicate that they had 

 arisen by a process entirely analogous to that of cell division. 



The nucleolar bodies lie at opposite poles of the nucleus but are always surrounded by 

 the nuclear substance and hence do not come in contact with the nuclear membrane. The 

 granules of the substance lying between them are disposed in straight lines which are 

 separated by tracts of clear protoplasmic substance, while from numerous points on the 

 periphery of the polar corpuscle the nuclear substance radiates either in the form of dis- 

 tinct fibres or rows of granules. (PL 20, fig. 33.) PL 21, fig. 47 is a section through the 

 nucleus of a blastodermic cell, exhibiting two nucleolar bodies, which lie within a space 

 free from nuclear granules or filaments. The bodies are separated by a thin layer of 

 nuclear substance, which is probably an optical section of the nucleolar plate. The body 

 of the nucleus is filled with fine tortuous filaments of nuclear substance. 



1 I cannot assert positively that the spindle figures which It is more than probable that such is the ease, since in a single 



I have seen in the nuclei of serosa cells of Oecanthus arise in preparation of serosa are to be found nuclei exhibiting all 



the same manner as in cells or produce the same effect upon the stages in the phenomena of spindle -formation and division, 



nucleus as they do upon the cells in other instances, since I Compare Flemming (1 7, 18), Priestly (37), Strasburger (39) 



have not observed the sequence of phases in any one nucleus. and the synopses of these papers by Mark (30). 



