398 H. H. Newman and J. T. Patterson. 



some distance from the latter do the individuals of a pair sever 

 their intimate connection and acquire separate amnia. Subse- 

 quently these embryos show their pairing in their mode of attach- 

 ment to the definitive placental discs, embryos I and II being 

 attached to the right hand disc and III and IV to the left. 



Fernandez calls attention in the case of the Mulita to the exact 

 identity in stage of development among the embryos of a set. 

 That this is not always the case in our species is well brought out 

 by a comparison of figs. 30 and 31, two embryos from vesicle 18. 



Fig. 30 represents embryo III, and IV was identical with it. 

 Fig. 31 was taken from embryo II but would serve equally well 

 as a figure of I. The difference in degree of development between 

 the two pairs is well marked not only in the number of somites 

 (5 in III and IV and 7 in I and II), but in the conditions in the 

 head region and in other parts. 



It is not likely that a difference in rate of development between 

 the two pairs is of common occurrence, but the clear case of it 

 just presented seems worth recording not only on account of its 

 rarity but because it serves to emphasize the tendency of the indivi- 

 duals of a pair to be alike, but somewhat different from the equally 

 identical opposite pair. 



Although of very common occurrence the pairing of embryos on 

 the basis of resemblances in the total number of scutes in the nine 

 bands of armor, is not without exception. In many cases the 

 pairing is so marked as to be startling, as for example in one case 

 where I and II each has 555 plates and III and IV each has 548; 

 or in another case where I and II have respectively 551 and 552 

 and III and IV have respectively 560 and 559. In many other 

 cases the pairing is obvious but not so clean cut. 



There are on the other hand two cases where there was a close 

 resemblance between three embryos, but one was strikingjy 

 different, as for example where II, III, IV have respectively 

 544, 545, 543 and I has 549; or again where I, II, III have 

 respectively 562, 565, 564 and IV has 573. Finally two cases 

 occurred in which, if any pairing at all exists it appears to be 

 between I and III and between II and IV, as for example where 

 I and III have respectively 544 and 546 while II and IV have 

 550 and 548. 



