FIG. 20. Stages in the development of Litomastix, after SILVESTRI (1906). 



a. Conjugation of egg and sperm nuclei. The dark vacuolated body is the 



" oosoma," and near the upper pole of the egg are the three groups of 



polar chromosomes. 

 by c. Two- and four-cell segmentation stages. The polar chromosomes unite 



into a single nucleus and the oosoma passes undivided into one daughter 



cell (b) and begins to break up (c). 



d. Fourteen-cell stage: the polar nucleus has divided into four; the oosoma 



has broken up and its fragments are contained in two cells which lag 

 behind the others in division. 



e. Twenty-cell stage : the fragments of the oosoma are contained in four 



cells ; the polar nucleus has divided into sixteen. 



f. Later stage. The polar nuclei and the protoplasm surrounding them 



have given rise to an envelope (trophamnion) surrounding the em- 

 bryonic mass. The latter consists of larger cells at the anterior and 

 smaller cells at the posterior end. The larger cells are probably derived 

 from those containing fragments of the oosoma. 



