426 RANDOLPH WEST 



vacuolated mesenchyme just lateral to the caudal muscle plate 

 and parallel to the axis of the notochord, and occasional very 

 small longitudinal spaces may be seen in the most lateral portion 

 of the caudal muscle plate. In one or two sections, one end of 

 the strand of myoblasts was seen to be in connection with the 

 muscle plate. Whether these cells were splitting off from the 

 muscle plate b}^ delamination, or whether they were forming 

 from the mesenchyme and being added to it by accretion, it 

 was not possible to determine in the material available. 



2. Morphogenesis 



Up to this point we have considered the histogenetic changes 

 which take place in the developing lymphatic plexus, and we 

 shall now consider the morphogenesis of the plexus. For this 

 purpose four wax reconstructions have been made by the method 

 of Born, three of which are here reproduced. 



Chick of 11 vim. Reconstructions of vessels and isolated spaces 

 of the caudal region. X 150. Figure 12: Arteries black, veins 

 and capillaries white, isolated spaces yellow. The postcardinal 

 vein and the aorta run a few sections above the upper level of 

 this reconstruction, but the coccygeal branches of the aorta 

 (fig. 12, 4) and a little more externally the coccygeal veins {3) 

 which drain into the postcardinals, are seen running downward 

 at right angles and dorsal to the axis of the vertebral column. All 

 of these structures are medial to the caudal muscle plate, which 

 has been omitted from this reconstruction for the sake of simplic- 

 ity. This muscle plate extends in a plane, parallel to the ecto- 



Fig. 10 Chick 12 mm., Series 46, Slide 1, Row 4, Section 4. X 500. Photo- 

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



Fig. 11 Chick 15 mm., Series 16, Slide 2, Row 4, Section 7. X 600. Photo- 

 micrograph of transverse section of caudal end of the embryo. 



Figure 10 Figure 11 



3, Coccygeal vein 5, Haemal capillaries 



6, Caudal muscle plate 17, Blood cells differentiating from 



16, Blood cells differentiating from lymphatic walls 



endothelium 



