191 



A year ago occasion arose for a brief elucidation of the pheno- 

 mena of like -twins, as examples of the development of two primary 

 germ- cells upon one blastoderm instead of but the usual single primary 

 germ-cell. A further consequence of the development of like -twins 

 would be some abnormality in the distribution of the primary germ- 

 cells; for, owing to the numerical law, to be presently set up, the 

 normal number, 2ii, would probably be distributed in some manner, 

 not necessarily equally, over two embryos instead of one. In such a 

 case the one embryo might be entirely sterile, and this would be the 

 probable explanation of the so-called "free Martin" in the ox^). 



Not until last spring has a case of like-twins upon one blastoderm 

 ever turned up among the eggs examined. Had such appeared earlier, 

 it would have been a curiosity, and nothing more! My researches of 

 previous years were not of a kind to permit of any particular use 

 being made of such rare specimens. Like-twins have now at last come 

 into my hands — just as they were wanted — not in one only but in 

 two instances. The four embryos obtained have not yet been sectioned, 

 and, therefore, an account of the finds must be postponed. They, and 

 the instances of certain monsters, to be presently described, are un- 

 doubtedly calculated to test, and in all probability to refute, the va- 

 lidity of current views of the formation of germ- cells from embryonic 

 tissues. 



Indeed, as it has turned out, the study of several monster-embryos 

 has served to discount the results to be obtained from the investigation 

 of the like-twins. Into the presumable causes of the arrestment of 

 these embryos it is not proposed to enter. Neither is it intended, 

 that a full account of all their peculiarities should be given here. 

 Their tissues were from the perfect state of preservation all alive at 

 the time of removal from the egg - capsules. No mitotic figures are 

 observable in them; but, as they are undoubtedly arrested in deve- 

 lopment, the cause of this deficiency is not to seek. Four of them 

 represent in degree of development 9 — 10 mm embryos, the fifth is 

 somewhat more advanced, perhaps 12 mm, while the sixth is some- 

 what younger, 6 mm. But, as a curious commentary upon the futility 

 of attempting to indicate their exact ages, it may be mentioned, and 



1) Vide Hensfn, Die Physiologie der Zeugung, p. 204. The facts 

 are as follows. Twin calves may be of unlike sexes, a male and a 

 female, or both female, in these instances they are usually normal. But 

 occasionally in cases of like twins in one chorion with one normal male 

 twin, the other is apparently sterile with abnormalities of its internal 

 and external sexual organs. 



