SEGMENTATION OF THE OVUM. 43 



count of the external appearances of the first segmentation in the living ovum of 

 the snail, Limax campestris. The eggs of this animal, by their size and in their 

 mode of segmentation, have a certain resemblance to mammalian ova. The fol- 

 lowing description is taken from the account by E. L. Mark, published in 1881; it 

 is nearly in his own words: 



In Limax, after impregnation, the region of the segmentation nucleus remains 

 more clear, but all that can be distinguished is a more or less circular, ill-defined 

 area, which is less opaque than the surrounding portion of the vitellus. After a 

 few moments this area grows less distinct. It finally appears elongated. Very soon 

 this lengthening results in two light spots, which are inconspicuous at first, but which 

 increase in size and distinctness, and presently become oval. If the outline of the 

 egg be carefully watched, it is now seen to lengthen gradually in a direction corre- 

 sponding to the line which joins the spots. As the latter enlarges the lengthening 

 of the ovum increases, though not very conspicuously. Soon a slight flattening of 

 the surface appears just under the polar globules; the flattening changes to a de- 

 pression (Fig. 7), which grows deeper and becomes angular. A little later the fur- 

 row is seen to have extended around on the sides of the yolk as a shallow de- 

 pression, reaching something more than halfway toward the vegetative or inferior 

 pole, and in four or five minutes after its appearance the depression extends com- 

 pletely around the yolk. This annular constriction now deepens on all sides, but 

 most rapidly at the animal pole; as it deepens it becomes narrower, almost a fis- 

 sure. By the further deepening of the constriction on all sides there are formed 

 two equal masses connected by only a slender thread of protoplasm, situated nearer 

 the vegetative than the animal pole; the thread soon becomes more attenuated and 

 finally parts. The first cleavage is now accomplished. Both segments undergo 

 changes of form; they approach and flatten out against each other, and after a 

 certain time themselves divide. 



The succeeding cleavages of segmentation need to be followed out in greater 

 detail than yet recorded. In many cases there appear to be three cells in the 

 next stage, because one of the two primitive segmentation spheres divides sooner 

 than the other. The more commonly received view is that four cells are produced 

 next, but it may very well be that there is really a three-cell stage preceding the 

 four-cell stage of which two figures are presented. The first of these (Fig. 8) 

 represents the four-cell stage of the ovum of a bat, and the second (Fig. 9) repre- 

 sents the four-cell stage of the ovum of the Virginian opossum. That of the bat 

 resembles the picture which we obtained from a number of animals, such as the 

 rabbit, the guinea-pig, the dog, and others. That of the opossum differs so much 

 from anything known in other mammals that it may be questioned whether it is 

 entirely normal. In the mouse the zona is much thinner and assumes an irregular 

 form, adapting itself to the pressure of the single spheres. 



After the four-cell stage, the segmentation proceeds apparently with considerable 

 irregularity, but we are soon able to see that the cells are grouping themselves 



