CURTIS: LIFE HISTORY OF PLANARIA MACULATA. 551 
process of fission soon reduces the adults to such proportions that 
they are indistinguishable from the rapidly growing young. Pre¬ 
sumably, the end products of asexual multiplication, being other¬ 
wise indistinguishable from the embryos of any year, will develop 
their reproductive organs.in the same way, but until the two have 
been isolated and reared there is of course no way of certainly 
ascertaining this. The following description is based upon speci¬ 
mens taken from sexual localities in the fall, and I have no way of 
telling what has been the previous history of any of them. How¬ 
ever, the study of some twenty-five or thirty specimens which had 
been cut into serial sections did not bring to light anything which 
led me to suspect two courses of development, one for the egg- 
embryo worms and another for those which are the end products 
of a season of asexual multiplication. 
In September, of 1899, I collected small specimens from 5 to 8 
mm. long in the vicinity of Baltimore, which upon examination 
showed their reproductive organs just developing. These stages 
have since been supplemented by specimens from the same locality 
and from the Falmouth ponds in the fall of 1900. 
The first indication of the developing reproductive organs is the 
appearance in the parenchyma, along a line just above either nerve 
cord, of rounded or cord-like masses of cells. Formative cells such 
as have been previously described (pi. 15, fig. 41 ; pi. 16, fig. 43, s) 
are abundant in this region and their frequent mitotic figures show 
that they are multiplying rapidly. Such a mass or cord of cells is 
shown in figure 46 (plate 16) as it lies surrounded by the paren¬ 
chyma. At this early stage there can be no question that these 
masses are made up of formative cells. At a slightly later stage, 
when one can distinguish the incipient testes (pi. 16, fig. 47) from 
the cords which are to form the yolk glands (pi. 16, fig. 44), the cell 
outlines are no longer visible, and although the general shape of the 
nuclei with the characteristic nucleoli (pi. 16, fig. 44-47) is the 
same as that found in the formative cells (pi. 16, fig. 43; pi. 
15, fig. 41), this alone would not be sufficient to establish the origin 
of the male and female elements from the formative cells. It is the 
abundance of such stages as figure 46 (plate 16) represents which* 
demonstrates beyond question the origin of the testes and yolk 
glands from the formative cells of the ventral region. 
The stages by which the solid rudiments of the testes (pi. 16, 
