The germ-cells. 6(35 



fied with germ-cells, the former term will no longer Ije used in the 

 present writing. Indeed, there is slight advantage in its retention in 

 embryology. 



Germ-cells of the sizes already indicated occur in various embryos 

 in most remarkable places. No two embryos are alike in this resjiect. 

 Such peculiar positions of these cells have long been known to me; 

 in fact, long before their nature was evident. Two of them were 

 figured and described in one of my memoirs ^) as lying in the brain 

 and spinal cord. Other observers have recorded them in positions 

 almost as unusual. But in embryos treated with the ordinary stains 

 as a rule only a mere fraction of these remarkable cells in unusual 

 places is seen. 



They are rare in the nervous system, much rarer than in other 

 places. Not very often are they found in the skin (Figs. 6 and 13). 

 None have ever been seen in the notochord, but they may occur almost 

 anywhere else. In certain phases there are always some in the gut- 

 epithelium (Figs. 25, 30, and 31) and the pericardium and its neigh- 

 bourhood practically always harbour a detachment (Figs. 27 and 51). 

 They represent the so-called segmental gonads of the "gono-nephro- 

 tome", that is, some of them are in the segmented mesoblast, especi- 

 ally of embryos under 10 mm. These, however, form a mere fraction 

 of the total. 



They may be found in any part of the mesoblast of the trunk ; 

 more frequently in early embryos they are between the layers, just 

 under the epiblast or between splanchnopleure and gut (Figs. 7, 28, 

 29, 36 etc.). They are sometimes represented very far forward in the 

 head; but their number here is not great, and there is no constancy. 

 They never occur in the tail. In one embryo only has a little group 

 been seen in the lumen of the cord in the region of the neurenteric 

 canal. These cells had undoubtedly got there at an early period, and 

 had been carried back by the growth of the tail. 



In neuroepithelia and in the thymus they have not been en- 

 countered ; but these are structures represented in early embryos by 

 small "placodes" or plates of cells, and each such plate has probably 

 arisen from a single cell. 



How do the germ-cells get into these positions? As germ-cells, 



1) J. Bbaed, The History of a transient nervous apparatus in 

 certain Ichthyopsida, Part I, Raja batis, in: Zool. Jahrb., V. 9, Anat., 

 figs. 69 — 70. 



