kofoid: development of limax. 80 



possible that a histological differentiation can have already taken place 

 between the two poles of the egg whereby the cells of the animal pole 

 are 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 

 distinct central cavity present; it becomes difficult in such cases to 

 locate cell boundaries and the relation of nuclei to them. In Plate III. 

 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 

 cavity, 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 

 47, a section of an embryo of eighty cells, and likewise in Figures 48 

 and 49 (Plate VII.), where the embryo has assumed the flattened shape 

 characteristic of the stage preceding gastrulation. In this egg the cavity 

 is small and lies between the ectoderm, the entoderm, and the 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 



