STRUCTURE AND DEVELOPMENTAL RATE 213 



their ventral surfaces, the lower figure, and finally ends with the 

 entirely separate unquestionably identical twins illustrated in 

 plate 4. 



The case of separate twins differs in many ways from the other 

 two and will be considered alone. 



The first two specimens are in general alike. In each the two 

 components are practically of equal size, and all of the four 

 components present entirely normal structures. These speci- 

 mens follow exactly the rule stated in reference to the double 

 trout; that is, when the two components of a double individual 

 are equal in size they are both normal in structure with almost 

 the same frequency as a single individual would be. 



In the first specimen, of plate 3, each head shows a perfectly 

 formed face with all the sense organs fully developed. A dis- 

 section of this body shows the vertebral columns to be separate 

 down to the sacrum. The pelvic skeleton is single with the nor- 

 mal single pair of lower extremities arising from it. The median 

 arms of the two components have their soft parts fused or pecu- 

 harly arranged, a synbrachium, the details of which are being 

 studied by Mr. H. B. Sutton. Each arm possesses a complete 

 skeleton. Further details of the visceral structures are of no 

 importance in the present connection. This specimen is in 

 general comparable to the fourth case of anterior duplicity shown 

 in the equal component series of trout (plates 1 and 2) . 



The second human specimen, in plate 3, is a case of full-term 

 united negro twins. There are two completely formed compo- 

 nents with a very wide ventral union into which the two livers 

 and other viscera are drawn. The babies are females and each 

 would weigh more than 5 pounds. A careful examination shows 

 all organs and parts to be perfectly formed and of normally 

 large size. This specunen is comparable to the last ones shown 

 in the equal component series of trout (plates 1 and 2) . 



Here again we are warranted in attributing the different 

 degrees of doubleness to the different distances apart of the two 

 original embryonic lines or axis as they appeared on the blasto- 

 derms. In the first case the primitive streaks were not far 

 apart and in the second case they were in positions almost 180° 



