Harris Hawthorne Wilder 413 



bryonic development of this the first two blastomeres may be supposed 

 to have shown the normal interrelation. 



A. II. — A case of duplicity in certain of the parts of the face and 

 head, resulting from a slight separation of the first two blastomeres in the 

 region ultimately to become the anterior end of the embryo, after which, 

 through the disturbed relations of these parts, each separate portion 

 attempted to develop both sides. 



Ex. — Moreau's case. Buffon's cat. 



[Lesser degrees of doubling have been mentioned above in connec- 

 tion with the cases just cited.] 



A. III. — Here the original separation was sufficient to involve the 

 •entire future head region, and each component blastomere, relieved of the 

 normal stimulus of contact with the other upon one side, has regenerated 

 the other half and thus developed an entire but duplicate head. 



Ex. — The two-headed turtle of Barbour. Externally Gruber's " Case 

 II " of Thoracogastrodidymus seems to belong here but the trunk is par- 

 tially double. It forms the link between this and the next, as a little 

 less separation would place it here, a little more in the next section. 



A. IV. — This stage is the first in the series to show the significant 

 phenomenon of a double median limb. In this and in all such cases 

 the limb is a bilaterally symmetrical member, the two lateral surfaces 

 being the same aspect (usually dorsal) and bearing the structures char- 

 -acteristic of the aspect presented. The structure bears so much the 

 appearance of a fusion or coalescence of two originally separate 

 components that many writers, perhaps the majority, speak of them 

 as such. This latter view is, however, manifestly impossible, for, grant- 

 ing that two members, once separate, could ever unite in such perfect 

 juxtaposition as to form a symmetrical piece, it would still be a serious 

 problem to dispose of the surfaces in contact with each other, and to 

 atrophy each component at such an equal rate that the result would 

 keep the perfect symmetry which these phenomena show. In the dia- 

 gram there is seen to be a doubled inner arm, contributed to equally by 

 each component. 



Ex. — To illustrate this case we are fortunate in having specimens, 

 carefully described and in part dissected, which represent a graded 

 series. Of these, the first is Gruber^s " Thoracogastrodidymus I," which 

 differs from his " Case II" of like name (A. Ill) merely in the presence 

 of a small median rudiment containing a double scapula with a pair of 

 conical acromion processes which together form the skeleton of the free 

 •end. Meckel's case possesses a larger median rudiment with a humerus 

 30 



