72 Florence R. Sabin. 



10, except that an opening cannot be made out. It is impossible to 

 say whether the opening is not present or whether the shrinkage of 

 preservative is sufficient to conceal it. 



The sixth embryo of the series (ISTo. 144), 14 mm. long, shows 

 only one new point in the formation of the jugular sac, namely the 

 beginning of the process of bridging of the sac which is illustrated 

 for a later stage in Fig. 14. This cutting of the sacs by slender con- 

 nective tissue bridges, which has already been described in the devel- 

 opment of the jugular lymph sacs in the pig, is, I believe, the 

 beginning of the transformation of the sac into a lymph node.^^ This* 

 will be considered later. 



Two of the embryos of the series, one (No. 350) measuring 15 mm. 

 and the other CNo. 106) measuring 17 mm., have very abortive sacs 

 near the junction of the primitive ulnar with the jugular vein. In 

 both instances the preservative is not good enough to show the en- 

 dothelium, so there is no way of telling these small sacs, which 

 measure less than half a millimeter in their longest diameter, from 

 mesenchyme spaces except by their position in comparison with other 

 embryos. They certainly are an evidence that there are marked 

 irregularities in the development of the lymphatic system. 



Another embryo of 15 mm. (No. 423) has also only a small sac, 

 this one measuring .9 mm. In the collection there are some embryos 

 in which the preservative is too poor to admit determining the 

 lymphatics at all, but out of the series of 22 which have been studied 

 there are four cases of abortive jugular sacs, or 18 per cent. These 

 embryos measure 15, 17 and 20 mm. 



The next specimen (No. 424), measuring 17 mm., is valuable, 

 for it has a double vascular injection. An extravasation along the 

 jugular region interferes with a study of the jugular lymph sacs, 

 but the vascular injection of the posterior part of the embryo gives 

 conclusive evidence that the other sacs, namely the retroperitoneal, the 

 posterior and the receptaculum chyli have not begun. 



Embryo (No. 296) measuring 17 mm. is the earliest specimen in 

 which I have found a valve undoubtedly open. This is shown in 



"Sabin. Amer. Jour. Anat. Vol. IV, 1905. 



