KOFOID: DEVELOPMENT OF LIMAX. 89 
possible that a histological differentiation can have already taken place 
between the two poles of the egg whereby the cells of the animal pole 
aro set apart to perform an excretory function. This is rendered still 
more doubtful by the frequent occurrence of eggs in which these secon- 
dary intercellular spaces have reached an enormous development at both 
poles, in fact throughout the whole egg. This condition may occur as 
early as the twenty-four-cell stage. In such eggs there is never any 
distinet central cavity preseut; it becomes difficult in such eases to 
locate cell boundaries and the relation of nuclei to them. In Plate ILI. 
Fig. 26, is shown a transverse section of such an egg containing more 
than one hundred cells. In stainability and nuclear conditions this is 
not essentially different from other eggs ; several cells of this egg are in 
a mitotic state ; I therefore believe such eggs to be normal. As can be 
seen in the figure, the three germ layers are present, and the vacuolation 
surrounds the cells of all three layers indifferently. There is no central 
cavity, and the three layers retain their connection with one another. 
Indeed, this condition is very suggestive of that found in the gastrula at 
the time when the head-vesicle is beginning to develop and the entoder- 
mal cells are sending out long processes into the fluid-filled space toward 
the cells of the other layers. It seems therefore no misuse of terms to 
designate the intercellular spaces in both cases as the primary body 
oavity, which throughout the period of segmentation is also the cleavage 
cavity. The condition represented in the figure is ephemeral and the 
extrusion of the liquid contents may take place without the formation 
of a spherical central cavity, The spaces seem to be thoroughly 
connected with one another and when some point on the periphery 
of the egg yields to the pressure, the fluid is probably in large part 
eliminated, 
The occurrence of a single distinct central cavity is shown in Figure 
4T, a section of an embryo of eighty cells, and likewise in Figures 48 
and 49 (Plate VIL), where the embryo has assumed tho flattened shape 
gastrulation. In this egg the cavity 
characteristic of the stage preceding 
is small and lies between the ectoderm, the entoderm, and tho bilaterally 
placed mesoderm bands. There is no trace of any cavity in the meso- 
derm. In Figure 54 the cavity occupies a position at the posterior end 
of the blastopore, and, as in the preceding stage, lies next to the ecto- 
derm on the dorsal side of the embryo. I have found many embryos, 
not figured, which have this definitely limited central cavity. In no 
case, however, has it attained the size of the cavity in the twenty- 
four-cell stage shown in Plate V. Fig. 34. On the other hand, a large 
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