375 
Soon after the two furrows were formed they extended around the 
upper pole of the ovum, cutting off the upper ends of the remaining 
octants (45) till they met; thus completing the 4th or Equatorial 
plane. The arrows in Fig. III show the directions in which they 
extend. 
Whether the departure from the usual mode of segmentation in 
Petromyzon described above, is in the case of P. marinus an abnorma- 
lity due perhaps to some unseen set of influences, or whether it is the 
regular method employed by the spe- 
cies, ıs I think still open to question. Fig. III. 
Considering the wonderful regularity 
with which the two planes in question 
occurred, the balance of favor seems 
to be for the second supposition; but 
then again as abnormalities are liable 
to occur so frequently in the early 
stages of segmentation one must be 
guarded against deciding too quickly. 
It is a well know fact that tem- 
perature influences the character of 
the segmentation, that low tempera- Fig. III. Semi-diagramatie9. 1, 2 
tures retard and high temperatures "1? same as in preceding figures. 
4 Furth plane of cleavage. 4a Indi- 
hasten the development and at the cate the place where the 4th plane of 
same time frequently affect the order cleavage first begins to show itself. 
in which the furrows occur. It might 
under certain conditions be possible to regard these irregularities for 
the normal course of development, as Calberla did in case of the first 
plane of cleavage. But when the question of temperature is elimi- 
nated as it certainly is in the experiments cited above, there is no 
other agent so far as I know that could have such an uniform effect 
upon the development of the ova, as to cause them all to segment in 
exactly the same manner, that isin a manner which was different from 
the one regularly employed. 
If the above results had been obtained solely from experiments 
on the first two sets of eggs which were fertilized under conditions 
very similar to each other, objections might well be taken. But since 
ova from six additional females were fertilized by as many different 
males, in different vessels and under varying degree of temperature, it 
9 It was thought best to represent this stage diagramatically because by the 
time the 4th plane of cleavage is complete, the Blastomeres have assumed a some- 
what irregular arrangement, rendering it less clear in making out the different 
planes and their relationship to each other. 
22* 
