704 
Yendo .— The Development of 
their appearance. The number of holes varies very much according to the 
habitat of the plant; in some cases there is a considerable number and 
in others only a few. 
The mode of formation of the holes is essentially similar to that in 
Agarnm Turneri , which has been studied by Humphrey. 1 In Costaria , 
however, the points of formation are not elevated on one side so remarkably 
as in Agarum. In a majority of cases one surface is slightly depressed and 
the other much more deeply (Fig. 28, PI. LIV). The tissue at this point, 
with both surfaces thus drawn together, is gradually atrophied. The 
epidermal layer of the less depressed surface lasts longer than the other, so 
that a round, milky-white remnant of membrane may be often still found 
stretching over a small hole. The healthy borders of the two epidermal 
layers come together as the atrophied tissue melts away, and finally the 
scar is entirely healed. The youngest holes visible to the naked eye 
measure about 2 mm. in diameter. They are widened as the blade extends 
and may reach often as much as 1 cm. in diameter. 
At the beginning of a post-embryonal stage the five ribs are evenly 
distributed in the lamina, dividing the entire breadth into six longitudinal 
strips. But in more advanced stages the marginal regions are far wider 
than the other intercostal areas and the lateral two ribs are more approxi¬ 
mated to one another than to the midrib (Fig. 29, PI. LV). 
II. Undaria. 
Embryonal Stages. 
The early stages of development of Undaria , from the filamentous 
sporelings until about 2 mm. in total height, are in the essential points 
similar to those of Costaria. The cellular arrangement, however, so far as 
the specimens in my hands have shown, is not so regular as in the latter 
case. Even in the blades comparable with the stage shown in Fig. 4, PI. LI 11 , 
the whole arrangement of cells could hardly be schematized in a simple geo¬ 
metrical figure. A frond measuring 0-9 mm. in total height, with a very 
short stipe and roundish blade—aberrant from the normal shape—had the 
cells in an especially disturbed condition. 
The precortical layer makes its appearance when a plant has attained 
17 mm. in total height. The mode of formation of this layer is quite 
similar to that in the case of Costaria. The tristromatic area, so far as any 
difference can be made out, is extended longitudinally, while attaining 
comparatively less width. 
The young fronds a few centimetres high of Costaria and Undaria are 
hardly separable one from the other by the naked eye. But in Undaria as 
1 Humphrey : On the Anatomy and Development of Agarum Turneri. Proc. of Amer. 
Academy, 1886, 
