THE DEVELOPMENT OF ORGANISMS 



723 



reproductive organs arise in connection with the middle layer or 

 mesoderm, and usually on an area of epithelium lining the coelom 

 or body cavity. The general fact is important, that the gonads 

 arise from patches of unspecialised germinal cells, which have not 

 shared in building up the body of the parent, and have retained the 



PB2 



PBl.- 



V 



^ 



51 



W 



id 2& 531 4a 



W 



V \A 



'msi 



V 



^a 



N 



\rTV 



W& 



Md 



Ha 



Fig. III. 



Oogenesis (to left) and Spermatogenesis (to right). After Boveri. I, Primary 

 oogonium; II-IV, its multiplication into oocj^tes. la, Primary mother- 

 sperm-cell; Ila-IVa, its multiplication into spermatogonia and sperma- 

 tocytes. V, A ripe oocyte gives off, by meiotic division, the first polar 

 body (PBi), which may divide mitotically. VI, The reduced ovum gives 

 off, by mitotic division, the second polar body (PB2), and becomes the 

 matured egg-cell (VII) or 4. Va, A spermatocyte divides meiotically into 

 two spermatocytes (Via) with the haploid number of chromosomes; and 

 these divide mitotically into spermatocytes which become ripe sperma- 

 tozoa. Note the theoretical presence of four cells (1—4; ia-4a) at the end 

 of each lineage. 



whole inheritance intact in the nuclei of their undifferentiated cells. 

 In other words, the germ-cells in the gonads belong to a lineage of 

 undifferentiated cells, unlike the somatic cells which exhibit division 

 of labour as muscular, nervous, glandular, skeletal, and so forth. 

 It is said that (unspecialised) somatic cells in the frog may give rise 

 to germ-cells, but, if this is the case, it is very unusual. Another 

 general fact of importance is that the early stages of ovaries and 



