502 CHARLES H. SWIFT 



occur in the chick, and, therefore, have studied with care the en- 

 toderm and splanchnic mesoderm, and in so doing have neglected 

 the key to the situation, the blood vessels. 



The embryo with 25 somites may be called a transition stage, 

 for the primordial germ-cells are present both in the blood ves- 

 sels (figs. 3 and 4) and in the splanchnic mesoderm. The embryo 

 with 21 somites may also be placed in this category. Were it 

 not for the presence of primordial germ-cells in the blood ves- 

 sels, this embryo with 25 somites could be passed over rapidl}", 

 because the position of those in the tissues coincides exactly with 

 their situation in the older embryos. That is to say, they are 

 found in the splanchnic mesoderm near its junction with the 

 somatic mesoderm at the coelomic angle. 



In the blood vessels the primordial germ-cells are found in two 

 situations, in the small vessels of the medial portion of the splanch- 

 nic mesoderm (figs. 3 and 4) caudad to the 19th somite in certain 

 vessels of the head (fig. 5). 



The primordial germ-cells found in the latter situation — that 

 is, in the vessels of the head region of this 25 somite embryo — - 

 are remarkably disposed. Twelve embr^^os of about this age — 

 between 22 and 29 somites — were examined, yet in onh^ one, that 

 possessing 25 somites, were the germ-cells massed. And, in- 

 deed, in no embryo studied was a like condition encountered. 

 The arrangement, then, of the germ-cells is probably abnormal, 

 but nevertheless, merits a description. The germ cells are pres- 

 ent in the radicles of the anterior cardinal veins (fig. 5). In this 

 region, as was mentioned before, the germ cells are not found 

 singly as in the vessels of the splanchnic mesoderm, but in masses 

 or lumps. These masses are four in number; a large group on 

 either side of the forebrain and two smaller groups. These col- 

 lections of germ-cells seem to entirely occlude the vessels in which 

 they are contained and remind one of emboli (fig. 5). Mixed in 

 with the germ-cells are the smaller blood cells (fig. 5). In one 

 of the large groups four of the germ-cells are dividing; this 

 presence of mitotic figures is worthy of notice, since in only 

 one primordial germ-cell (fig. 10), these excepted, was division 

 observed, from their origin until they pass into the gonad. To 



