474 



"with germ-cells, the former terra will do longer be used in the present 

 writing. Indeed, there is no 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 respect. 

 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. 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, and 

 the pericardium and its neighbourhood practically always harbour a 

 detachment. They represent the so-called segmental gonads of the 

 "gono-nephrotorae", that is, some of them are in the segmental meso- 

 blast, especially 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. 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 

 umen 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 encoun- 

 tered; 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, 

 formed before the embryo arises, their business is to migrate into the 

 embryo, when this is formed. Perhaps sometimes they start too soon, 

 or too far forward, or they may take the wrong path. If they start 

 too early, they may get landed and stranded in the nervous system 

 or skin; if too far forward, they come to lie, it may be, far in front 

 ^f the future genital region. It must be noted, that their paucity in 

 ihe head-region, and their absence in the tail both indicate that their 



