512 PROCEEDINGS OP SECTION D. 



and in part fused. Miss Duncan, of Aberdeen (4), has described a 

 similar case of posterior duplicity in a chick embryo, under the title of 

 " The Anatomy of a Double Chick Embryo." In both these cases there 

 were two, or portions of two, primitive grooves upon a single ovum. 



2. Laguesse and Bue (5), in an account of a monster with two heads 

 and one body, measuring only 19mm. in length, state that they think 

 that there were first two primitive lines close to one another, and slightly 

 curved, with their convexities directed towards one another. At the 

 extremities of these two lines were developed two distinct cephalic 

 prolongations, two neural canals, and two notochords. Up to this point 

 it would have been possible to have had a pair of twins from this con- 

 dition had the two primitive lines been sufficiently distant from one 

 another. But so close were they that there was between them poste- 

 riorly only a narrow territory of common blastoderm, so that in the 

 sacral region, though there were two cords, there was only a single 

 cartilaginous vertebral column. 



3. Duval (6), speaking to his theory of polyspermia, states that 

 following upon the polyspermia would be the apparition of two primitive 

 streaks, which may take any position with regard to one another — at 

 right angles, opposite, side by side, &c. — and consequent upon their 

 relative position to one another will be the position and amount of fusion 

 of the future double monstrosity. 



The present author is, therefore, of opinion that double terata 

 result from the evolution, upon a single ovum, of two primitive grooves 

 produced without bilateral segmentation of that ovum. 



As to why two germinal areas should be induced in a single ovum, 

 the most generally acceptable theory is that of Duval. In this theory 

 polyspermia is the chief, if not the only, cause of redundancy of develop- 

 ment. This theory is accepted, apparently in its entirety, by Laguesse 

 and Bue in the article previously quoted, whilst Ballantyne (2) himself 

 says : " The theory of polyspermia certainly affords a very good work- 

 ing hypothesis to account for the various finds of double terata." 



The theory of polyspermia is not, however, universally recognised 

 as affording the correct explanation of the occurrence of two germinal 

 areas upon a single ovum. Schultze (7), for example, rejects the theory 

 in favor of the idea that diplogenesis is caused by some condition of the 

 ovarian ovum before impregnation, which condition he believes to be 

 an incomplete cellular division. Another argument which has been 

 adduced against polyspermia as the cause of double terata, and which 

 has never been satisfactorily answered, is this — if the entrance into a 

 single ovimi of two spermatozoa can produce double monstrosities, why 

 cannot three spermatozoa so gain access and produce triple monsters, 

 and so on ad infinitum. 



Arguments such as this are difficult to answer, for in man no ocular 

 proof can be adduced either in favor of or against polyspermia ; but, 

 these adverse views notwithstanding, it seems probable that polyspermia 

 is a determining cause of double terata. That irregular development 

 does follow polyspermia seems to be practically proved by the obser- 

 vations of Gemniill, (8) who, in a valuable paper on the " Vitality 

 of the Ova and Spermatozoa of certain Animals," says : " It is a 



