PROCEEDINGS OP SECTION D. 513 



remarkable fact that polyspermia and irregular development are more 

 apt to occur in the case of ova that are fertilised immediately after 

 being shed into sea-water than in those which have been left to them- 

 selves for a little, e.g., for from one to four hours before fertilisation is 

 attempted. Apparently the short interval gives them time to round 

 off their circumference, and allows their peripheral layer to become 



accustomed to the medium of sea- water As is well known, 



in the normal fertilisation of the sea-urchin egg, immediately after the 

 entrance of the first spermatozoon head, the peripheral layer of the 

 ovum throws off a delicate, but firm, membrane — ' the membrane of 

 fertilisation '^which effectually prevents the entrance of additional 

 spermatozoa. In well-marked cases of polyspermia this membrane is 

 never formed. It is to be noted, however, that when perfectly fresh 

 ova are taken from urchins that are perfectly ripe immediate fertilisa- 

 tion may be effected without the occurrence of more than occasional 

 cases of polyspermia. But whenever the ova are taken from a gland 

 that is either immature or half spent, more cases of polyspermia occur 

 with immediate than with delayed fertilisation." 



Gemmill's remarks seem not only to suggest that polyspermia 

 is a determining factor in irregular development, but also to show that 

 the condition of the ovum has to be taken into account in the determina- 

 tion of monstrosities. It may, therefore, be that cases of triple mon- 

 strosities are unknown, because the ovum, being a mononuclear struc- 

 ture, cannot possibly tolerate for germinal processes more than two 

 spermatozoa ; consequently double monstrosities only occur. On this 

 theory — w^hich is merely put forward as a tentative reply to the argu- 

 ment as to why three spermatozoa do not gain access to an ovum, and 

 so produce triple monstrosities — triple monstrosities would result in 

 the event of polyspermia occurring in a polynuclear human ovum, a 

 contingency so improbable as to be altogether beyond the laws of 

 nature. 



It seems to me, therefore, from pe 'sonal observations and from a 

 study of the works of others, that double terata result from polyspermia 

 occurring in a mononuclear and single ovum, the ovum itself playing 

 in an as yet undiscovered way some pathological part, and that fertilisa- 

 tion having occurred in this way development proceeds in the single 

 ovum without bilateral segmentation. 



Passing now to the consideration of the embryological errors which 

 resulted in the various abnormalities in the case now recorded, it may 

 be stated that the intestinal variations of the single duodenal tube, the 

 single jejunum, and the hepatic and pancreatic variations are due to 

 the development of the twins from a single ovum, and with but one 

 yolk-sac between them as already fully explained in the paper by Berry 

 alreadv referred to. 



The explanation of the very remarkable cardiac variations is more 

 difficult, but is probably as follows : — The description which has already 

 been given of the apparently single heart shows that it is in reality a 

 question of the partial fusion of two originally distinct tubular hearts. 

 The fusion of the two inferior cav;c to form a single structure prior to 

 its termination in the right auricle of the right twin show that this 

 k2 



