74 PATTEN. [VoL. XII. 
median, unpaired organs, which in their serial arrangement 
follow the same order as in the metamere itself, namely, a, 3, 
Grae, 
It is thus obvious that the rate and direction of growth is 
such that each new half tends to form a right-angled triangle, 
the apex of which coincides with the posterior end of the 
embryo, the altitude with the mid-ventral line, and the base 
with the width of the oldest half-metamere. 
We thus see a gradually increasing series of new organs 
appear along the altitude of the triangle, or the old mid-ventral 
line. They attain their full size and perfection of form for 
that stage; then each divides into two (the one a mirror image 
of the other), which move away from the median line, and in 
their former place appears a new unpaired set, composed of 
the organs that normally lie lateral to the ones just formed. 
The successive ‘eruption of new series of organs along this 
median line, and the manner in which they divide and move 
away from it to right and left, is so entirely different from 
what we have been accustomed to see that it is very impressive. 
This effect is not diminished on further reflection. 
Examination of the diagrammatic figure illustrating an 
incomplete double embryo shows that each metamere has a 
lateral growth similar to that which occurs at the posterior end 
of segmented animals. Jz posterior, apical growth a number 
of like parts, or segments, tncreasing in age and in differentia- 
tion toward the anterior end, is produced. In the lateral growth 
of a half-metamere a series of unlike parts ts produced. Lut 
while there is a similar method of growth in both cases, there ts 
in the second case a greater increase of specialization in passing 
Jrom the growing point toward the part first formed. 
If we carry the process seen in Pl. VIII, Fig. 90, back to its 
beginning, we are led to conclude that fission began by the for- 
mation of new organs in the median line at the very anterior 
end of the body, that is, in the indentation separating the right 
and left semicircular lobes of the brain. The first organs to 
appear then must have been the cephalic lobes. But each half 
of a cephalic lobe consists of at least two parts, a lateral one, 
the optic ganglion, and a median one, the semicircular lobe and 
