On Reproduction, Animal Life Cycles. 83 
with his determinant theory, Weismann has been obliged to assume 
the transmission of reserve-determinants to whatever somatic cells 
may regenerate or bud. Such an argument is generally recognized 
as highly hypothetical and even improbable; it is the weakest side of 
the whole Weismannian theory. In our present discussion it is 
wisest to leave out of account the assumed determinants and to try 
to reason upon a more empirical basis. This has been done by 
Curtis , 9 who refers normal fission to the energies of undifferentiated 
embryonic cells that persist throughout the adult body, or occur at 
least in the greater part of it. This idea seems to me quite justi- 
fiable; and it is clearer and simpler than other explanations because 
instead of appealing to reserves of determinants or of referring the 
problem to polarity, it reasons from the existence of cells that can 
be empirically studied. In the Tuybellaria the cells of the so-called 
parenchyme appear relatively undifferentiated, and occur through- 
out the body beneath the outer skin. From a similar tissue in the 
Nemertines, but which I preferred to call mesenchyme in order to 
show its similarity in function and origin with the similarly named 
tissue of Vertebrates, I found 10 that the germ cells appear to arise 
from a simple connective tissue that is of diffuse arrangement. We 
may simply enlarge the idea of Curtis and consider all such undiffer- 
entiated cells to be in potentia germ cells, and that such cells are the 
initial agents in all asexual reproduction. They are cells that have 
taken no part in the formation of definite gonads, but which at the 
same time have not become somatically specialized. In this connec- 
tion the important discovery may be mentioned, made by Farmer, 
Moore and Walker , 11 and confirmed by Walker , 12 that the leucocytes 
may contain the reduced number of chromosomes, and even the form 
of the latter characteristic of the germinal chromosomes of the first 
maturation mitosis. This is an important instance of cells not placed 
in gonads exhibiting a characteristic of germ cells that has hitherto 
been supposed to be limited to definitive germ cells. 
On this point of view, budding would differ from sexual repro- 
duction mainly in this respect, that the point of departure of the 
new individual is a single or a number of immature germ cells (ones 
that are not ovotids or spermatozoa), and that these immature germ 
cells carry with them some somatic cells. Up to this time, so far as 
my memory serves me, the difference between these kinds of re- 
9 The Life History, the normal Fission and Reproductive Organs of Plan- 
aria maculata, 1902, Proc. Boston Soc. Nat. Hist., 30. 
10 On the Connective Tissues and Body Cavities of the Nemerteans, with 
Notes on Classification, 1897, Zool. Jahrb., 10. 
“On the Cytology of Malignant Growths, 1896, Proc. Roy. Soc. London, 77 . 
“Observation on the Life-History of Leucocytes, 1906, Ibid, 78 . 
