The Developnieut of Phascolosoma. 8| 



2. Development of the Germ Cells. 



The ovaries and testes each consist of a fringe- shaped cord 

 which runs ti-ansversely across the base of the ventral retractor 

 muscles near their orig-in, and extends across the mid-ventral line 

 beneath the ventral nerve trunk. The g-enital cord consists, as 

 Andrews (1890) and others have shown, of irregular masses of germ 

 cells, held together by fibres of connective tissue, and surrounded by 

 coelomic epithelium. A delicate narrow band of the same supporting 

 tissues unites the genital cord to the retractor muscles. The primi^ 

 tive germ cells (Fig. 1) lie in the interior of the cord, the irregular 

 surface of which is covered with masses of cells about to become 

 detached from it. 



The oogonia, which become detached from the ovary in the 

 female, measure 25-30 ^i in diameter. These cells are set free either 

 singly or in masses, and are covered with coelomic epithelium, derived 

 from the ovary, which forms a partial follicle. 



The oocytes increase in size in the coelomic fluid to five or six 

 times their usual diameter, i. e. to 150—180 //. During this period 

 of growth the nucleus occupies a central position in the egg, which 

 at this time shows no indications whatever of polarit}-. The chro- 

 matin throughout this period is in the form of numerous spherical 

 nucleoli of different sizes (Fig. 2), which are scattered irregularly 

 through the nucleus, suspended in the linin network. These spherules 

 of basichromatin, or pseudonucleoli, increase to a certain degree both 

 in size and in number during the growth of the egg, but they do 

 not become transformed during this period, so far as my observations 

 have extended, either into a network or into a spireme. Numerous 

 fine granules of oxychromatin (as shown by their reaction to eosin) 

 appear suspended in the linin network of the youngest eggs. In 

 half-grown oocytes this material may become massed into true 

 nucleoli of a transitory character. 



During the period of growth a deeply -staining granular 

 layer appears in that part of the cytoplasm which immediately sur- 

 rounds the nucleus (Fig. 2). This layer gradually increases in thick- 

 ness, and extends nearly to the periphery of the cytoplasm. I regard 

 this as an indication that rapid metabolism is going on in the region 

 around the nucleus. Distinct radial fibres extend in half-grown ova 

 (Fig. 2) from this inner layer of densely granular cytoplasm outward 

 to the periphery through a layer which appears less compact, since 



Zool. Jabrb. XXIII. Abt. f. Auat. 6 



