TRANSACTIONS OF SECTION K. 757 



4. Formation of the True Ntideoli or Plasmosomes of the Somatic Cells: A 

 Contribution to the Study of the Formation of the Plasmosomes in the 

 Nerve and Mood Cells of some Batrachians, viz., Rana temporaria and 

 Alytes obstetricans. By Professor T. Havet. 



The plasmosome of somatic cells is usually circular in shape and situated 

 within the nucleus. It consists of a central part which stains with acid dyes (acid 

 fuchsine, eosin, Congo red, orange, &c.), and a peripheral granular poi-tion 

 coloured by basic stains ; ex. hsematoxylin. 



Nothing is known as to the formation or origin of the plasmosome, except that 

 it makes its appearance during the telophase and disappears at the prophase. 



In my researches regarding the origin of the true nucleoli I have carefully ob- 

 served all the details of the different phases in their formation in the nerve, blood, 

 and connective-tissue cells of the tadpoles of frogs and toads, and have obtained the 

 following results. The true nucleolus makes its appearance at the very com- 

 mencement of the telophase as a small clear area, which does not take the colouring 

 reagents. It lies in the centre of the telophasic figure, and can only be seen when 

 this figure is viewed from its summit. The chromosomes thus examined are 

 found to be piled up against one another (the polar aspect) and arranged radially 

 around the central clear area. The chromosomes then become vacuolated and 

 form themselves into a network, which first appears in the external portion of the 

 telophasic figure, while the internal portion of the latter (i.e., that in proximity 

 to the clear central zone) remains thickened. Gradually the alveollsation and 

 network formation extend inwards towards, but not reaching quite up to, the 

 central area. The internal extremities of the chromosomes do not become 

 vacuolated, but persist as a circle of granulations or chromatic blocks around the 

 clear central area. This area has in the meanwhile become slightly enlarged, 

 owing probably to the enchylematic fluid, which at this period is found in 

 abundance. 



The study of the formation and origin of these plasmosomes furnishes a good 

 indication of their structure. The central plasmatic portion, coloured by the acid 

 dyes, being constituted of a more or less modified nuclear enchylema, while the 

 peripheral basic staining portion is derived from the internal extremities of the 

 concentrated, non-vacuolated chromosomes. 



It should be remarked that the central portion often contains a kind of net- 

 work, formed by a chromatic substance of nuclear nature and staining with 

 hsematoxylin, the peripheral zone having the same structure and origin as 

 described above. 



Whence arises this special structure ? It sometimes happens that a chromo- 

 some, instead of behaving like the others equidistant from the central zone, 

 becomes included and traverses this central area, where it also undergoes vacuola- 

 tion, thus giving rise to the chromatic reticulation in the central area of the plas- 

 mosome. 



"When, from some unknown cause, the chromosomes unite during the telophase 

 into several (usually into two) bundles, their behaviour is the same, except that 

 they are arranged around two (or more) clear areas instead of one ; hence the 

 explanation of the occurrence of several plasmosomes within a single nucleus. I 

 have frequently noticed the presence of two such bodies. 



It is well known that at the prophase — that is to say, at the moment of the 

 recondensation of the chromatin and of the reconstitution of the chromosomes — the 

 nucleolus disappears. The peripheral zone of the plasmosome is derived from a 

 portion of the chromosomes, as is also the remnant of the chromatic network, and 

 both furnish material for the reconstitution of the chromosomes. The central part 

 of the plasmosome, which is merely nuclear enchylema more or less modified, 

 becomes mingled with the surrounding nuclear enchylema. Hence the explanation 

 of the disappearance of the nucleoli, which has been the object of my study. 



It is easily understood what happens at the prophase in the case of the plasmo- 

 some having a chromatic network within its central zone. 



