170 EDWARD MCCRADY, JR. 



have been brought together, actually they are farther apart 

 than ever before and the mouth cavity internally is quite large. 



Mammary anlagen. The cross-cut surface in figure 54 

 shows an epidermal thickening between the midline and the 

 base of the hindlimb. Medially the thickening projects deeply 

 into the mesenchyme, and on the surface of the body it lifts 

 up slightly above the surrounding epithelium. This is the 

 anlage of a single mammary gland. There is no continuous 

 mammary rige at first, but these separate anlagen arise in 

 pairs which are arranged into two lines. The most anterior 

 pair appears first, and is situated just craniad of the level of 

 the umbilicus. The more caudal pairs appear in succession 

 forming two lines of separate anlagen which converge toward 

 the phallus. There are usually six such pairs forming a sort 

 of horseshoe, and in addition, a single, unpaired, median 

 nipple at about the level of third nipple from the posterior 

 end in the lateral series. Fairly often there are additional 

 pairs at the anterior ends of the lateral series so that the total 

 may be fifteen or seventeen. In the case of one embryo 

 (17173) I have found the 'median' nipple also paired, making 

 a total of eighteen. In adult females the most anterior pair 

 is sometimes lost, leaving only eleven glands in all. The 

 average number, however, is thirteen. These appear in both 

 male and female embryos, and sex will not be distinguishable 

 in any way until stage 35. 



Connection betiveen thoracic duct and jugular sacs. In 

 stage 33 a medial sprout from the jugular lymph sac on either 

 side was described as growing toward the region of the 

 thoracic duct. And it was mentioned that at the same time 

 separate lymphatic anlagen of perivascular origin were form- 

 ing in the mediastinum in two longitudinal rows which di- 

 verged anteriorly from the median, single portion of .the 

 thoracic duct. By the coalescing of the perivascular lymphatic 

 anlagen and the medial sprouting of the jugular sac extensions 

 the thoracic duct attains connection with both jugular sacs 

 in stage 34. This makes its anterior end complete, but pos- 

 teriorly there is no cisterna chyli as yet. 



