VIVIPAROUS FISHES OF THE PACIFIC COAST. 
441 
constituent biopbors are scattered* at the time or soon after its extrusion from the 
germinal vesicle. 
Significance of the yolk nucleus . — A yolk nucleus, metanucleus, Nebenkern, of one 
description or another, has been observed in the eggs of all groups of metazoa 
exclusive of those of the Porifera and of the Echinoderms. While it has been 
observed in such a variety of animals, the explanations it has received have not been 
commensurate with its distribution. It is true that a yolk nucleus has not been 
observed in species whose near relatives have this structure well distinguishable. It 
very frequently disappears soon after its formation, and we need only go a step farther 
to a condition when it is distributed through the cytoplasm during its formation, and 
from this condition there is but a step to the separate extrusion of its constituent 
parts. This may explain its absence in species whose near relatives have it. 
While in many eggs it appears early, in others ( Forskalia ) it is not formed till the 
time of maturation. In all cases in which its formation has been traced it originates 
from the nucleus as something cast out without the usual formalities of cell division. 
Its function has been supposed to be that of yolk formation, but it is found in some 
eggs in which yolk is never formed or, after all the functions of the egg as a cell, aside 
from its hereditary functions, have disappeared. It has been supposed to give rise to 
the follicle, but it sometimes does not appear till the follicle has begun to degenerate. 
It has been supposed to represent the male element in the egg, and in the case of 
parthenogenetic ova to replace the spermatozoon, and thus has been attributed with 
the function later assigned to the second polar globule. This last explanation may 
have a grain of truth in it, but it is far from being satisfactory. 
Moreover, while attempts have been made to homologize every other structure or 
action of the egg with a structure or action in the spermatozoon, I am not aware that 
this body has received the same distinction. And yet there arises in the spermatozoon 
a body called by the same name (Nebenkern) in very much the same manner. 
* While the theory of the germplasm is admirably delineated, some objections to the ideas of a 
biophor as described by Weismann (The Germplasm, Am. Ed. 1893) may be added here. These objec- 
tions may be answered by the statement that Weismann is not endeavoring an explanation of life. 
But it is fair to insist that the definition of the theoretical units should not exclude the possibility 
of life. 
■ The biophor is said to be the smallest unit which exhibits the primary vital forces, viz, assimila- 
tion and metabolism, growth and reproduction by fission. The difference between biophors of 
various kinds “depends on either the absolute relative number of molecules, their chemical constitu- 
tion (isomerism included), or their grouping.” Both suppositions can not hold. For instance, a par- 
ticular biophor may be composed of 7 (or any other number) of molecules, 3 of one sort, 3 of another, 
and 1 of a third sort. It would be impossible for such a biophor to give rise by division to another 
biophor containing the same number of molecules of the same sort arranged in the same manner. 
We must imagine that this biophor during growth appropriates molecules like those composing 
it until double the original number are present of each particular sort of molecules. Adivision into two 
similar halves would thus be made possible, but the character of the biophor, according to the sen- 
tence quoted above, would have been changed with each molecule added. Or, we must imagine that 
all the molecules necessary for the formation of a new biophor are appropriated simultaneously, in 
which case the biophor would suddenly enlarge to double its normal size. This sudden growth might 
indeed be the agent causing fission. This last alternative verges dangerously near forbidden ground — 
i. e., the formation of new biophors outside of the original biophor through the simple presence of 
the latter, after the manner Nageli supposed new micellae to arise. 
