DEPARTMENT OF EMBRYOLOGY. 107 



The work done on cytology in this department may be grouped 

 under two headings — descriptive and experimental. The descriptive 

 work comprises papers by Professor Jules Duesberg and by Mr. 

 Norman Chve Nicholson. It should be added that Miss Thurlow's 

 paper, reported last year after the manuscript was read at the meeting 

 of the Anatomists, has since appeared in full, with figures, in the Con- 

 tributions to Embryology of the Carnegie Institution (No. 226). 



Professor Duesberg, applying the methods for chondriosomes to 

 fish embryos, found these bodies in all the cells of such embryos from 

 the segmenting stage up to the time of hatching. In the first blasto- 

 meres the chondriosomes are gi'anular (mitochondria) and are espe- 

 cially numerous in the marginal cells. Later, filaments (chondrioconts) 

 are found almost exclusively. Dr. Duesberg's paper contains a de- 

 tailed description of the chondriosomes in the cells of the epiderm, 

 central nervous system, gangUa, chorda, mesenchyme. Wolffian ducts, 

 and digestive tract; also in blood corpuscles, myoblasts, and primordial 

 germ-cells. It was found that in mitosis the shape of the chondrio- 

 somes was somewhat modified, inasmuch as frequently the filaments 

 break into smaller pieces, and that they become accumulated in the 

 anaphase between the daughter-nuclei, as observed in cells of embryos 

 of other vertebrates. These observations show that the structure of 

 the cell in fish embryos is essentially the same as in other groups. The 

 protoplasm is formed of a ground substance, appearing homogeneous 

 in the preparations, and of variously shaped chondriosomes. 



Mr. Nicholson has studied the qualitative variations in the mito- 

 chondria of nerve-cells in white mice. He finds that the variation 

 in morphology between cells of different varieties is often quite pro- 

 nounced. Filamentous mitochondria constitute the most common 

 form met with in the cells of the central nervous system. They are 

 particularly apparent in the large anterior horn-cells and in the large 

 cells of the reticular formation. Rod-like and granular mitochondria 

 are more rare, but are characteristic of the cells of the mesencephalic 

 nucleus of the fifth nerve as well as of the cells of the Gasserian gan- 

 glion. The cells of the nucleus of the corpus trapezoideum may be 

 distinguished by their large, swollen, block-like mitochondria. There 

 is also, in the majority of cases, a variation in the form of mitochondria 

 in different parts of the same cell. For instance, they are usually more 

 granular in the vicinity of the nucleus than in the peripheral parts of 

 the cytoplasm or in the processes. In the latter they are invariably 

 rod-hke or filamentous. The cells of the nucleus of the trapezoid body 

 constitute a special case, because in them the mitochondria always 

 occur in the form of long blocks in the peripheral cytoplasm, in sharp 

 contrast to the minute, granular, and rod-Hke mitochondria in the 

 immediate neighborhood of the nucleus. The mitochondria not only 

 occur between the Nissl bodies (as is generally believed), but are 

 embedded in them. 



