The origin and histogenesis of the thymus in Raja batis. 425 



appear later on, Stieda's memoir contains no contribution at all 

 towards the solution of the problem. And now there is no longer 

 any difficulty about the matter, when dozens of drawings and thou- 

 sands of preparations can be produced to show it. 



Where Nature places no obstacle in the way of carrying-out a 

 process, it is not for Man to invent one ! The fact is, that in nature 

 as KÖLLIKER first stated in 1879, adenoid tissue does arise from 

 epithelial cells. 



yi. The Thymus-Placodes in Embryos of 24—27 mm. 



In embryos of 23 mm and smaller the production of leucocytes 

 is not great, and their emigration from the placode can rarely be 

 observed. And , as we have seen , there are comparatively few 

 leucocytes within the latter. Immediately after this period, in embryos 

 of 24 mm or more, progress is being made. Leucocytes are far more 

 numerous within the epithelium, from 20 to 30 being met with in a 

 single transverse section (Figs. 7 and 15). Their emigration is now 

 a more conspicuous phenomenon. One may still meet with single 

 leucocytes, caught in the act of emigration ; but this has begun to 

 give place to a slightly different process. 



For Raja hatis the period of 24 — 27 mm is an important one, 

 not only as relates to the thymus-placodes, but also with regard to 

 other structures. In series of sections it is represented in the col- 

 lection by more than a dozen embryos. Of these eight have been 

 drawn upon for one or more figures. The embryos and figures are: 

 No. 633 (Fig. 49), No. 201a (Figs. 15, 22, 23, and 24), No. 201 

 (Figs. 7 and 47), No. 202 (Figs. 11, 12, 14, 37, and 38), No. 202a 

 (Figs. 11 and 12), No. 203 (Figs. 16, 39, and 42), No. 443 (Figs. 31 

 to 34, 35a and b, 36), No. 616 (Figs. 45 and 46), No. 629 (Fig. 44). 



The topographical relationships of the placodes during this period 

 may be gathered from text-figure F. 



Skate-embryos of circa 25 mm exhibit the following among other 

 characters. External gills are present on all five branchial arches, on 

 the last two they are but short. Forked muscle-buds from about 46 so- 

 mites are passing into the paired fins. Unpaired fins are evident, 

 first appearing in embryos of about 24 mm (preserved), but they con- 

 tain no mesoblast. The neurenteric canal is closed. The lateral line 

 extends some Uttle distance beyond the last gill-cleft, but only a short 

 way along the pectoral fin. The spiracle is elongated. There is no 

 upper jaw. The olfactory organ is a simple pit. 



