412 . RANDOLPH WEST 



cularization of the tissue accompanied by a marked decrease in 

 the number of blood cells in the tissue spaces. It seems highly 

 improbable that decided haemopoesis should take place only to 

 let the cells formed disintegrate three or four hours later without 

 having entered a vessel, and moreover none of the blood cells 

 observed in the tissues appeared to be disintegrating. Mc- 

 Whorter and Whipple (11) in their study of the chick blasto- 

 derm in vitro have observed a to-and-fro movement of the 

 blood cells in the tissue spaces synchronous with the heart beat, 

 and have also observed the entrance of these cells into the 

 general circulation following their rhythmical movement. This 

 phenomenon might be regarded as a plasmatic pulse, which 

 would eventually force any blood cells lying free in the tissue 

 spaces into the general circulation. In addition those cells 

 having the power of amoeboid movement could enter the vessels 

 by diapedesis through the capillary walls. 



B. DEVELOPMENT OF THE LYMPHATIC PLEXUS AND ACCOM- 

 PANYING HAEMOPOESIS 



The changes about to be described take place only in the 

 mesenchyme lateral to the caudal muscle plates in the posterior 

 region of the embryo, the mesenchjane lying medial to the 

 muscle plates maintaining its compact indifferent character. 

 For the sake of clearness we shall first consider the Histogenesis 

 and then the Morphogenesis of the developing plexus of lymphatic 

 vessels. 



Fig. 1 Chick 8.5 mm., Series 21, Slide 1, Row 3, Section 2. X 200. Photo- 

 micrograph of transverse section of caudal end of the embryo. 



1, Notochord 5, Haemal capillaries 



2, Neural tube 6, Caudal muscle plate 



3, Coccygeal vein 7, Differentiating blood cells 



4, Coccygeal artery 



