416 RANDOLPH WEST 



the bounding cells do flatten, renders it highly probable that a 

 similar process takes place in the case of the smaller spaces which 

 fii'st acquire a venous connection. 



As was pointed out by E. R. Clark (12) at the Christmas meet- 

 ing of the Anatomical Society in 1913, there are present in the 

 mesenchyme lateral to the caudal muscle plate in the posterior 

 region of the embryo certain strands of flattened cells which 

 Clark holds to be outgrowths from the venous endothelium and 

 to be always capable of being traced back to the veins. These 

 cells, he says, contain nuclei which may be distinguished from 

 the mesenchyme nuclei by their morphological and staining 

 characters. 



That strands of flattened cells, sometimes with continuous 

 lumina, sometimes with an interrupted lumen or with no lumen 

 at all occur in the chick as early as 9.5 mm. and more abun- 

 dantly in the later stages, is true. But that they can be clearly 

 distinguished from mesenchyme cells, and that they can always 

 be traced back to a venous endothehum, are at least open 

 questions. 



E. R. Clark (12) describes the endothelial nucleus as being 

 rather pale and elongated with one or two definite reddish 

 discoid nucleoli, while the mesenchymal nucleus he holds to be 

 darker, and more chromatic with one or two irregular bluish 

 nucleoli, not sharply differentiated from the surrounding chro- 

 matin material. A careful examination, however, reveals a 

 series of graduated stages between these two forms of nuclei. 

 A slight change in the focus of the microscope will make a bluish 

 nucleolus appear reddish, and vice versa, while a careful study 

 of the tissue reveals great variance in the amount of chromatin 



Fig. 3 Chick 15 mm., Series 16, Slide 2, Row 4, Section 8. X 300. Photo- 

 micrograph of transverse section of the caudal end of the embryo. 



), Notochord 7, Differentiating blood cells 



^. Neural tube 8, Mesenchymal space 



3, Coccygeal vein 9, Lateral branch of coccygeal vein 



i, Coccygeal artery 10, Lymphatic connected with vein 



0, Haemal capillaries 11, Aorta 



6, Caudal muscle plate 



