166 OTTO FREDERIC KAMPMEIER 



throughout its total extent, the sinus is found to be nowhere in 

 connection with the veins, although it closely approaches the 

 external jugular of each side at its extreme anterior hmit; nor has 

 it as yet established junction with the lymph hearts. 



Plate 4 is representative of conditions observed in a 7 mm. 

 embryo. The upper sketch, figure 10, drawn from a section of 

 the region eventually occupied by the completed sinus, shows 

 this vessel as a double or bilateral rudiment (I.) which does not 

 communicate from side to side, but each half is isolated from the 

 other and consists of several individual anlagen which are de- 

 veloped ventro-laterally of the external jugulars (j.d. and j.s.). 

 In the lower picture, figure 11, is sketched a highly magnified 

 area on the left including the vein (j.d.) and three lymphatic 

 anlagen (I., I.) in cross section. One of these is solid, being 

 laden with yolk globules, and the larger two are hollow but 

 possess walls which are dense and firm. With reference to yolk 

 content, the drawing, a faithful copy of the actual state of affairs, 

 impresses the distinction between mesenchyme and lymphatics 

 so forcibly that further words to the same effect are superfluous. 

 Indeed, there are occasional mesenchymal elements which do 

 contain yolk, but then it is usually in the form of minute granules 

 and is distributed thinly in the protoplasm. Other distinguish- 

 ing marks are not so apparent. In all probability the lymphatic 

 endothelial nuclei are on the whole somewhat smaller and more 

 compact than those of the connective tissue at this relatively 

 late stage, but manifold exceptions are encountered among them 

 and consequently the observer would hesitate to emphasize such 

 a difference unduly. 



Turning to the next previous stages, 6 and 5 mm. embryos, 

 more interesting phenomena were witnessed, which are in part 

 reproduced on plates 3 and 2. Figure 7 was drawn from a trans- 

 verse section of a 6 mm. specimen to exhibit the features in the 

 environment of the left .external jugular (j.s.). At this level 

 the larger one of the two sinus anlagen (L) contains a small 

 slit-like lumen, the confines of which are thick and are packed 

 with yolk spherules. This lymphatic followed from end to end 

 was discovered to be applied to the lining of the vein at its ante- 



