1892.) Embryology. 785 
into normal structures. In the bony fish the tendency to the forma- 
tion of double-headed monsters would be due to the method of closure 
of the blastopore, two invaginations being easily brought together to 
form a common trunk. In the chick, however, this cannot so readily 
take place, but embryos arising peripherally on the blastoderm tend 
to have their heads fused while the tail ends are not brought together 
by the fusion of any growing edge forming the blastopore and so 
remain separate. 
This leads to the consideration of the conditions producing double 
germs from a single egg. A single egg after the first cleavage has the 
power to produce two individuals of normal structure but half the 
normal size. This is the necessary result of the process of cell divis- 
ion as previously explained by the author, and has recently been 
shown experimentally by Driesch, Chabry and not really negatived by 
Roux, when his work is interpreted as seems just. 
The first two cells of a cleaving oosperm develop into right and 
left halves only because of their association together; separated each 
would form a perfect organism. 
The reason for the manifestation of this double power in double 
monsters is to be sought in the action of forces before cleavage. Of 
these the author regards polyspermy as the most efficient. This view 
the author upholds in spite of the many negative experiments that 
have been made upon echinoderm eggs (and upon frog eggs in the 
present paper, granting that polyspermy actually took place in the 
frog’s eggs used). 
Here it may be noted that the author assumes throughout that the 
frog’s eggs were injured by the treatment he gave them, and that more 
than one sperm entered each abnormal oñe. 
There is, however, no evidence of this in the present paper; we find 
only a certain similarity between the treatmont of the eggs and the 
treatment of echinoderm eggs when polyspermy actually ensued. 
Back of the effects produced by entrance of many sperms there is 
the abnormal state of the egg allowing of this multiple fertilization. 
This state of the egg with the effects of polyspermy remain latent 
until later several invaginations may result and from these eventually 
double monsters are formed if there be not a complete fusion of the 
first rudiments. 
The connection between polyspermy and the formation of double 
monsters is thus by no means a direct nor a simple one, yet of the 
many factors concerned the effects of polyspermy are, in the author's 
estimation, the important ones. 
