Formation of Double Embryo in Hen'n-egg. 115 



into "nests" having the form of masses of })rotoplasm 

 containing nuclei, the " nests " becoming subsequently 

 constricted off into distinct cells, it might be supposed that 

 two nuclei became enclosed in a common mass of 

 protoplasm. In this case, even supposing that each nucleus 

 became united with a spermatozoon, we should expect to 

 find two distinct blastoderms formed, an occurrence which 

 has not as yet been noted. 



Thirdly, and most probably, it may be sup]50sed that the 

 very first division of the nucleus was abnormal. The nucleus 

 may have become divided into two halves, which were quali- 

 tatively and quantitatively precisely similar, and not, as we 

 may suppose to be the case in normal division, slightly 

 different from each other. Probably, in the normal develop- 

 ment of at all events the higher form of life, from the very 

 first division of the nucleus and protoplasm, parts are produced 

 which differ minutely from each other, and neither of which, 

 if placed in suitable circumstances, could give rise to a perfect 

 animal. Abnormally, we may su])pose that division takes 

 j)lace in such a way that the two half nuclei are exactly 

 similar to each other, when further develojnnent may, under 

 suitable conditions {e.g., a sufficient mass of nuclear material 

 remaining in each half, and a sufficient supply of nutrient 

 material), result in the formation of two similar and fully- 

 formed embryos. 



This suo-o'estion will also serve to account for the cases of 

 incomplete division of the embryo, as in the one figured by 

 Mr. Lucas. In these cases, abnormal segmentation, resulting 

 in the production of two halves precisely similar to each 

 other, only takes place at a later stage, and so only affects 

 certain cells (and their nuclei), which will give rise to certain 

 organs of the body. Hence, we get a duplicature of such 

 structures ; or, if abnormal division again follow, a still further 

 multiplication of them. The further back in segmentation, 

 tliat is the earlier this abnormal division takes place, the 

 larger is the part of the body affected, until, if the division 

 affect the segmentation nucleus of the ovum, the result is two 

 complete and separate embr3"os. 



I 2 



