Florence R. ^^al)in 367 



nuclei lie definitely within tlie tiyncytiuni and belong to the connective 

 tissue. They are only to be distinguished l)y the fact that they are in 

 clumps and that some show karyokinetic figures while others are smaller 

 and take the deep stain of a newly divided nucleus. In short, cell pro- 

 liferation takes place around the capillaries. 



Passing now to the bands or bridges of connective tissue, the first point 

 to be noted is that there are numerous blood capillaries filled Avith red 

 blood cells, many of them nucleated. The same sort of protoplasmic 

 network is present as in the surrounding tissue, but the network is denser 

 and the meshes finer. The increase seems to be in the granular proto- 

 plasm rather than in the fibrils. In this dense network of protoplasm 

 are crowded many connective tissue nuclei ; the mature ones are oval in 

 shape and take the stain faintly. Many of the nuclei are dividing, and 

 there are numerous small, round, deeply staining, young nuclei. These 

 round nuclei belong, however, to the connective tissue and there are no 

 true wandering cells outside of the blood-vessels. Thus the modifica- 

 tion of the tissue around the sac consists merely of an increase in the 

 blood capillaries, and in the connective tissue protoplasm and nuclei. 

 The cell increase does not take place independently of the blood capil- 

 laries. There is no muscle in the wall of the sac at any time. By the 

 time the embryo is 3.6 cm. long a second node is Just beginning at the 

 other end of the sac. This second node from the lymph sac develops in 

 the same manner as the first, but slightly later. 



We pass now to the primary lymph node when the embryo is 4.9 cm. 

 long, as shown in Fig. 9. The section is taken in the same plane as Fig. 8, 

 that is, it is from a set of transverse sections. The efferent ducts are 

 on the right and the hilum at the top of the section. There are no strik- 

 ing changes between this and the preceding stage. The node as a whole 

 has increased considerably in size. The lymph heart is about the same 

 actual size as in Fig. 8, but the lymphatic plexus is greater. From Fig. 1 

 it can be seen that when the ducts first start out from the sac they grow 

 directly to the skin, but in Fig. 9 there has been an anastomosis or plexus 

 formation of the ducts on the border of the sac. This greatly extends 

 the area of the node. On the left side of the sac there are a few blood 

 capillaries with clumps of dividing nuclei around them. The bands of 

 connective tissues shoAv the same abundance of blood capillaries and in- 

 crease in the protoplasm and nuclei. Young and dividing nuclei are 

 abundant, Init no true wandering cells are present. 



A more important stage is met with when the embryo is 7 cm. long. 

 From this stage on, the development of the primary lymph node is shown 



