THE TRANSVERSE PROCESSES OF NECTURUS 541 



ber of cells between the elastica externa and interna. Between 

 the notochord and lateral muscle mass is found loose mesenchy- 

 mal tissue, in which at this stage a condensation is appearing 

 laterally, which extends outward into the horizontal septum 

 marking the position of the future rib (fig. 2, r.p.) and also up- 

 ward to the neural arch along the inner edge of the transverse 

 septum in the position of the future rib-bearer (fig. 2, r.b.p.). 

 In tracing this condensation mesally toward the notochord, it 

 will be seen, in the greater number of vertebrae, to become much 

 weaker and gradually dissolve into what appears to be typical 

 mesenchymal tissue. In some vertebrae the basal stump is 

 indicated by a few^ slightly modified mesenchymal cells which 

 line up between the notochord and the rib proton- (fig. 2, p.a.p.). 

 In either case the greatest condensation of mesenchyme appears 

 laterally at the inner margin of the lateral muscles, and weakens 

 toward the notochord. 



Goeppert's belief in the rib as a lateral outgrowth of the basal 

 stump and the rib-bearer as a dorsal outgrowth of the same 

 element were based apparently upon the mistaken observation 

 that these structures are continuous in the proton stage. The 

 examination of a number of vertebrae at this stage of development 

 shows that there is a difference in the time of appearance of the 

 protons of these structures. The rib and rib-bearer protons 

 are well marked out in many of the vertebrae before the proton 



Fig. 2 Transection through the fourth vertebra of a 20-mm. larva, n.a., 

 neural arch; n.c, notochord; p.a.p., proton of parapophysis; r.b.p., rib-bearer 

 proton; r.p., rib proton; v. a., vertebral artery. 



Fig. 3 Transection through the fourth vertebra of a 20-mm. larva shghtly 

 more advanced in development than that shown in figure 2. n.a., neural arch; 

 p.a., parapophysis; r.b.p., rib-bearer proton; r.p., rib proton. 



Fig. 4 Transection through the fifth vertebra of a 21-mm. larva, h.a., 

 haemapophysis; n.a., neural arch; p.a., parapophysis; r.b.p., rib-bearer proton; 

 r.p., rib proton. 



Fig. 5 Transection through the eighth vertebra of a 21-mm. larva, h.a., 

 haemapophysis; n.a., neural arch; p.a., parapophysis; r.b.p., rib-bearer proton; 

 r.p., rib proton. 



2 The term 'proton,' introduced by Prof. B. G. Wilder, is used in the present 

 commvmication in preference to the word 'anlage.' It has the same meaning and 

 has the added advantage of being an English term. 



