HISTORY OF CHROMOSOMAL VESICLES IN FUNDULUS. 253 



on the details of the transition from one phase of mitosis to 

 another that the new conception obtained from the study of the 

 processes in Fundulus throws light, and at the same time offers 

 a "raison d'etre" for the appearances seen. 



Fundulus cleavage cells show nuclei which are favorable to 

 the study of these processes for the very reason that they are 

 small in relation to the large amount of cytoplasm present. 

 Because of the physical conditions thus engendered, the chromo- 

 somes and chromatic structures have a characteristic loose 

 arrangement which permits their close study. At the same time 

 this renders a study of individual chromosomes, as has been 

 done for instance in orthopteran material, impossible. The 

 behavior of a group of chromosomes as a whole is easily followed, 

 but individual differences as in size in metaphase chromosomes 

 are so slight that it is quite beyond the range of practicability 

 to follow a single chromosome through the various metamorphoses 

 it undergoes. 



The material used in this investigation consisted of Fundulus 

 eggs, fertilized as already stated by either Fundulus or Cteno- 

 labrus sperm. Eggs were preserved during the early stages of 

 cleavage (first five cleavages), and also during the later stages of 

 development, but the cells from the early stages are the most 

 favorable for cytological study, and it is chiefly from them 

 that the figures are drawn. Fixation was most satisfactory in 

 Bouin, although other fluids were also used. Iron haematoxylin, 

 variously counterstained, gave the best results. The blastodiscs 

 were removed from the yolk and embedded and sectioned 



separately. 



OBSERVATIONS. 



The metaphase plate may serve as a convenient point at which 

 to begin observations on the mitotic process. Fig. I shows the 

 conditions of chromosome and spindle relation in the normal 

 Fundulus eggs at the time of maximum condensation of the 

 chromosomes in the metaphase. It is from a blastomere in 

 the fifth cleavage division. The cytoplasmic mass of such a 

 blastomere is quite large as contrasted with the volume occupied 

 by the spindle, and of course is large in comparison with the 

 amount of chromatin present. This figure shows conditions 



