1888.] Mr M. Brown on Arrested Twin Bevelo^ment. 465 
6. On the Specific Gravity of the Water in the Firth of 
Forth and the Clyde Sea Area. By Hugh Robert 
Mill, D.Sc., Scottish Marine Station. 
7. Arrested Twin Development. By Macdonald Brown, 
F.B.C.S. 
(Abstract.) 
One of the most interesting departments of embryology is that 
which treats of twin development, and the many forms of abnormality 
or monstrosity produced by the arrest or modification of its process. 
When the first cleft of the ovum is incomplete at some part, and 
two organisms arise from its halves, arrested twin development in its 
widest sense occurs, and a form of double monstrosity results. That 
the condition is due to imperfect cleavage, and not to a fusion of two 
ova, is shown by the fact that the sex of the twins is invariably the 
same, and, as Ahlfeld has pointed out, that they are always united 
at identical parts. The individuals are rarely of equal size and 
perfection of structure, although there are several instances on 
record where such was the case ; more commonly one of the 
organisms has its growth arrested at an early period of life, and 
clings as a parasite to the fully developed autosite. In such cases 
the parasite varies in size and form, existing sometimes as an almost 
complete organism, possessing not only external parts, but internal 
organs; at other times, it is represented by a limb or part of such 
only (as in the case described by Handyside). This arrested 
development of the second organism (parasitism) is undoubtedly 
due to an incomplete blood supply passing to the second foetus, a 
condition brought about either by placental changes, or changes 
within the body of the monster itself, at a very early period of intra- 
uterine life. 
The recorded cases of parasitism are numerous, and from the 
highly illustrated and mythological work of Licetus, published in 
1634, to that of Ahlfeld in 1880, numerous articles and treatises 
have appeared, and various classifications have been formulated. 
After having described at some length the chief varieties of the 
condition, the author demonstrated a case of the thoracopagous 
form, which a few weeks previously had come under his notice. 
