SPERM FORMATION 



3 



father is older than the son, while if the 

 continuity of the germ cells be considered, 

 the son is one generation older than his 

 father. 



III. A Typical Anim.\l Cell (Fig. 1.) — 

 The germ cells are usually much larger than 

 the somatic cells and are thus used as a basis 

 for general study. All cells are made up of 

 a nucleus enveloped by a more or less viscous 

 substance, the cytoplasm. The substance in- 

 side of the nucleus is spoken of as nucleo- 

 plasm or karyoplasm. In the nucleus there 

 is usually found a heavy network (chro- 

 matin network) and a dense mass of chro- 

 matin, the chromatin-nucleolus. There may 

 also be found a finer network of achromatic 

 substance, (linin-network) and sometimes a 

 spherical mass, the plasmosome, of the same 

 material (Goldsmith '16). The nucleus to- 

 gether with the cytoplasm constitutes the pro- 

 toplasm. 



IV. The Method of Cell Division — After 

 the cell has reached a certain stage of 

 growth, the chromatin-network begins to 

 condense at various points in the nucleus — 

 usually near or in contact with the nuclear 

 membrane (Fig. 5). These aggregations be- 

 come more and more definite and individual- 

 ized until they each assume a compact and 

 characteristic form, (prophase stage, Fig. 6.) 

 These newly formed bodies are called chro- 

 mosomes and are uniformly characteristic of 

 the species in which they are found. They 

 vary from two in a thread worm to 1,600 in 

 a ceratin radiolaria. A small body (the 

 centrosome, (Fig. 7) from which radiates 

 cytoplasmic lines (astral rays Fig. 7), now 

 appears on either side of the nucleus. The 

 nuclear wall is then destroyed as prominent 

 fibers (spindle fibers Fig. 7) connect these 

 two points as centers and force the chro- 

 mosomes into a median position, thus present- 

 ing a typical spindle (metaphase condition 

 Figs. 19 and 7). The chromosomes now di- 

 vide lengthwise and one-half of each passes 

 to opposite poles of the spindle. While the 

 chromosomes are on their way to the poles 

 (anaphase stage Fig. 8) the entire cell begins 

 to divide. This continues until after the 

 chromosomes have reached the spindle (telo- 

 phase Fig. 9), when the two daughter cells 

 are completely separated. The chromosomes 

 of each daughter cell now reassume their 

 early woolly appearance and gradually pass 

 into the network condition, while the nuclear 

 membrane and the other parts of the cell are 

 being reconstructed in preparation for the 

 next divisions. 



V. The "Green" Germ Cells — (sperma- 

 togonia). The processes in cell division are 

 essentially the same wheth'er somatic or 

 germinal. However, since the former cells 

 are smaller, the component parts are more 

 compact and not so readily differentiated. 



From a very early embryonic condition until 

 near the time the animal is capable of repro- 

 duction, the large cells which were set aside 

 for this purpose, gradually increase in num- 

 ber by the above method of cell division. 

 (See plate "Green Germ Cells.") 



When the animal is near maturity (soon to 

 be capable of reproducing)^, a number of these 

 cells seem to lose their division energy. 

 They then enter a quiescent or rather a ripen- 

 ing stage in preparation for the great function 

 for which they were ordained, namely, that of 

 reproduction. We who live "microscopic 

 lives" speak of this stage as the growth, or 

 maturation period of the germ cell. The 

 speaker has one form under investigation in 

 which he is able to predict by the appearance 

 of the cells those which have about lost their 

 division energy and are ready to undergo 

 maturation. 



VI. The Ripening of the Germ Cells — 

 (The maturation period) A. The growth 

 stage. After an early germ cell has lost its 

 division energy and is ready for ripening, the 

 two daughter cells resulting from the last di- 

 vision fail to reconstruct as usual to form 

 another division spindle. The nuclear wall is 

 usually reformed but the nucleus itself is filled 

 with a dense fibrillar but somewhat granular 

 network (Fig. 2). Among this homogeneous 

 network there is usually (indeed, always in 

 the domestic animals) found one or more 

 compact bodies resembling the chromosomes 

 (Fig. 2 to 11). In fact, v/e have demon- 

 strated conclusively that these bodies have 

 persisted as distinct individuals from the 

 chromosomes of the earlier divisions, all 

 others having been spun out to make this 

 new network. These dense bodies never 

 break up as do the other chromosomes, but 

 continue as independent elements while the 

 cell is ripening. They are called the "sex- 

 chromosomes" or better "female producing 

 chromosomes" since they are, at least, asso- 

 ciated with sex-production. 



As the cell and nucleus continue to grow, 

 some very interesting changes take place in 

 the fibrillar network. Typicallj' it forms into 

 as many long strands as there were chromo- 

 somes in the earlier stages not considering the 

 sex-chromosomes (Fig. 3). The ends of these 

 dense threads are now mysteriously drawn to 

 one region, usually against the wall of the 

 nucleus. Since they seem to be floating in 

 a nuclear sap the median parts of each thread 

 swing out into the nucleus, forming a bundle 

 of very clear loops (Fig. 4). The cytologist 

 calls this the bouquet stage of the ripening 

 cell. The female producing chromosome is 

 usually in the thickest part of this bundle. 

 As these loops decrease in size, presenting a 

 very compact condition (synizesis) they are 

 ofttimes observed to pair (synapsis). Since 

 each loop represents a chromosome, the num- 



