448 l\Iabel Bishop 



before it continues backward as a single spinal cord. The larynges and 

 cesophagi are separate do\\Ti to the level of the shoulders (approxi- 

 mately). 



The last stage in the cosmobiotic series under discussion that I have to 

 present is represented by a two-headed turtle of the common variety 

 Chrysemmys picta, just hatched, traces of the umbilicus still remaining. 

 Its carapace is a trifle smaller than a twenty-five cent piece. Both 

 carapace and plastron show a beginning tendency toward doubling [see 

 Plate IV, Figs. 1 and 2, x and y]. Components A and B are disunited 

 as far back as the shoulder girdles. Two distinct hearts are present. 

 Upon removing the plastron and gently spreading the heads apart, a 

 median papilla is conspicuous between the two necks at the level of the 

 outer fore limbs, and immediately posterior to it in the mid-shoulder 

 region on the ventral side is a tiny median nodule of cartilage with 

 delicate muscles attached. The papilla is unquestionably the anlage of 

 a median pair of fore limbs, and the nodule is probably the beginning 

 of a median double shoulder girdle. Save for these peculiarities the 

 specimen appears normal in general structure. It represents a stage 

 between the dicephalus dibrachius and dicephalus tribrachius of the 

 human species represented by Forster [Tafel I, Figs. 10 and 11]. Ahl- 

 feld portraA's a condition more nearly like that of Teras XX [Tafel X, 

 Fig. 10], but Gruber depicts a stage practically identical with it [Tab. 

 Ill, Fig. 2] — with this exception, that in Gruber's specimen the conical 

 tuberosity is compressed somewhat from the front backward (page 34) 

 while in Teras XX the compression is lateral. 



It will suffice for the purposes of the present study to consider but one 

 of the many systems of these organisms. I have selected, therefore, 

 a system that is fundamental to the growth and development of every 

 being, and one that appears very early in embryonal life, namely, the 

 circulatory system. This is functional from its beginning and thus 

 reflects the physiological needs of an organism in every stage of develop- 

 ment. Only the arteries in the doubling region and the heart will be 

 considered in the present paper. In each teras described I shall con- 

 sider the heart and anterior arteries in detail and make comparisons 

 with those in normal specimens of the same species and of approxi- 

 mately the same age. I shall then compare these vessels in the cosmobia 

 themselves, and offer speculative explanations of the variations noted, 

 and endeavor to permanently establish the cosmobia in the series already 

 defined. 



