Early Development of Desmognathus Fusca, 539 



segmentation could be seen at the lower pole, that from living eggs 

 alone it would seem as though the development approached the mero- 

 blastic type, for it was seldom that any lines of segmentation were 

 found in early stages outside the light cap of formative yolk at the 

 animal pole. A presen-ed egg of a four-cell stage which has been divi- 

 ded into four partial blastomeres for some time is shown in Fig. 8 from 

 above and Fig. 9 from the side turned a little; it shows the rather 

 light second division not reaching to the edge of the white polar disc, 

 while the first division plane is not only heavier but also much more 

 extensive, reaching much below the white polar cap, on one side 

 nearly two thirds of the distance from the animal to the vegetable 

 pole of the egg and on the other side not quite so far, but at this stage 

 the cleavages have not progressed far enough to enable one to see any- 

 thing of them when the egg is viewed from the lower pole. Sections 

 of a four-cell stage show the first division plane to be simply a rather 

 shallow groove in the mass of yolk. The second segmentation plane 

 is found as a somewhat narrower and shallower groove. ISTo other 

 indications of division could be detected in any other part of the 

 egg. Both grooves were confined to the animal pole where the yolk 

 granules in something less than a third of the upper pole were much 

 finer than the rest of the egg (Fig. 49). 



The conditions so far described for the four-cell stage apply to the 

 earlier stages ; later these rather regular planes of division may be- 

 come irregular due to the shifting of the blastomeres or perhaps due 

 to the movement of the yolk granules. This shifting, which is indi- 

 cated in Figs. 10, 11 and 12, is often carried to an extreme so that 

 deep wide fissures may be formed between the blastomeres as in Fig. 

 10. In the egg of Fig. 11, the blastomeres have shifted considerably 

 and we have the beginning of an other division indicated by a small 

 branch off from one of the heavier lines. In Fig. 12, it will be 

 noticed that there are beginnings of other divisions indicated by 

 branches from the heavier lines. An interesting point in this stage 

 of development which perhaps may be seen better in more regiilar 

 and later stages, is the absence of anything which might be called 

 an equatorial division which is so characteristic of many amphibian 

 eggs. Fig. 14 shows quite well how the additional blastomeres are 



