214 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
undivided. A single nucleus was visible in the small segment, but tbe egg in reality 
contained six cells, live lying in tbe unsegmented yolk. 
It would be interesting to know bow many of tbe irregularly segmented cells 
eventually attain a normal condition. It seems probable that very many of tbein do, 
judging from tbe fact that tbe number of abnormal eggs wbicb later appear when tbe 
nauplius stage is reached is much smaller, yet there is no evidence that any of tbe 
eggs are lost. 
THE INVAGINATION AND EGG-NAUPLIUS STAGES. 
I will now speak of some interesting variations wbicb occur during tbe invagina- 
tion period and immediately after it. 
Instead of tbe normal ingrowth of cells from tbe surface into tbe yolk and tbe 
sinking in of others to form a small circular pit, there is what appears at tbe surface as 
a deep transverse invagination. This is sometimes a long crescent-shaped or irregular 
transverse fissure, as in the egg of wbicb cut 40 represents a median longitudinal 
section. 
In other cases, in wbicb tbe processes of development have gone further, there is 
formed an irregular, oval, or circular disk of cells in connection with the invagination, 
as shown in fig. 229, plate 51. Here there is a well-defined rim on one side, while upon 
tbe other tbe structure seems to blend with tbe yolk. In a further-developed stage in 
the same process I find that tbe egg has often a well-defined, sometimes round, and 
very irregular circumvallate disk of cells. Tbe cells within tbe vallum are densely 
crowded, and tbe presence of numerous karyokinetic figures shows that at times cell 
division may become rapid. Below tbe surface, both within and without tbe vallum, 
tbe granular masses of chromatin bear abundant testimony to tbe degeneration of living 
protoplasm wbicb is taking place in tbe yolk. Tbe columnar aspect of tbe marginal 
cells of tbe disk can be plainly seen. Tbe way in wbicb this condition is reached is 
illustrated by cuts 39 and 40. By the ingrowth (or infolding in consequence of unequal 
growth) of some of tbe superficial or ectoblastic cells into tbe massive ball of yolk, a 
tongue-shaped or island-like patch of cells is formed, on wbicb tbe embryo proper is 
subsequently marked off (figs. 228 to 231). 
Tbe egg-nauplius may arise in a depressed central part of tbe disk, as in fig. 231, 
or upon its margins, figs. 228, 230. 
We will now glance at tbe histology of some of tbe abnormal embryos. Out 40 
shows a median longitudinal section through one of tbe earlier stages described. 
When tbe egg was examined from tbe surface a transverse irregular fissure was seen, 
corresponding to tbe pit {Pit) where tbe sheet of cells dips below tbe surface. We see 
from a study of this egg that a considerable stratum of cells, including the invaginate 
area, has grown into tbe yolk, and that its edges are folded upon themselves. In this 
case one side of tbe disk, corresponding to tbe anterior end of tbe embryo, is at tbe 
surface, while tbe opposite side is deeply embedded in tbe yolk. Numerous cells have 
budded off from this cell plate, particularly at its posterior end, where they multiply 
rapidly and move about freely in the yolk, like tbe normal mesendodermic cells. Like 
tbe latter, they move chiefly in a posterior direction into tbe deeper parts of tbe yolk. 
Many of these wandering cells are moreover already in process of degeneration. It 
looks as if there was a migration of cells from tbe surface behind tbe cell plate, but 
tbe appearances may be in this respect deceptive. Tbe yolk flows over tbe engulfed 
cells, but I find in my preparations no new superficial layer of ectoderm established. 
