390 ROBERT W. HEGNER 



the formation of the first polar spindle. The nurse chamber 

 (n.c.) contains a syncytium with about eighteen large nuclei, 

 each of which possesses a large, centrally placed nucleolus sur- 

 rounded by irregular chromatin granules. At the posterior pole 

 of the egg is an accumulation of material (pPl) which stains 

 deeply, and, as will be shown later, is intimately associated with 

 the origin of the primordial germ-cell. This mass of material 

 has been termed by Kahle 'polares Plasma'^ — a term adopted in 

 the following description. A discussion of the origin and signifi- 

 cance of the 'polares Plasma' will be reserved until later (p. 396). 



There is nothing unusual in the process of maturation. The 

 egg nucleus preparing for division is shown in figure 29, m.s. 

 One polar body is formed in Miastor metraloas and the number 

 of chromosomes could not be determined by Kahle but is prob- 

 ably from twenty to twenty-four. The number of chromosomes 

 appears to be similar in M. americana but an accurate count 

 could not be made. The first polar body divides by mitosis 

 (fig. 30, p.b.) and the pronucleus (/.n.) moves over into the mass 

 of cytoplasm (c), which is apparently elaborated by the nuclei 

 in the nurse chamber, and becomes the cleavage nucleus. Here 

 in this mass of cytoplasm the first cleavage takes place resulting 

 in two apparently similar daughter nuclei (fig. 31, c.n.). 



Reference must be made to Kahle 's figures and description 

 for most of the events of early cleavage. The two nuclei of the 

 two-cell stage divide by mitosis and the four daughter nuclei 

 are apparently similar. They have been numbered i, ii, iii, iv, 

 beginning at the anterior pole (fig. 3). The succeeding nuclear 

 division is important, since the advance from the four-cell to the 

 eight-cell stage witnesses the origin of the primordial germ-cell 

 as well as a casting out (diminution) of chromatin by nuclei i, 

 II, and III. In figure 3 nuclei i, iii and iv are shown in mitosis. 

 Spindle iv is undergoing the ordinary process of mitosis, but 

 spindles i and iii (and also spindle ii, which is not shown in the 

 figure) are long and slender and a large portion of their chromatin 

 (fig. 3, cMp) does not take part in the formation of the daughter 

 nuclei but remains behind in the cytoplasm as a 'Chromosomen- 

 mittelplatte.' These masses of cast-out chromatin are never 



