EARLY STAGES OF LIMULUS. 223 



i. Cleaveage. 



About forty-eight hours after fertilization, broad furrows, filled with small 

 spherical bodies, appear on the upper surface of the egg. There is no regularity 

 in the direction of these cleavage planes, or in the form of the blastomeres, 

 although a four cell stage resembling that of many amphibia, is frequently seen. 

 (Fig. 126, c.) 



Cleavage is most marked on the surface that happens to be uppermost. 

 Eggs with ten or twelve prominent blastomeres on the upper surface may be per- 

 fectly smooth and apparently unsegmented on the under side. However, when 

 stripped off the glass and inverted, segmentation appears on the smooth under 

 surface in about fifteen minutes, and in half an hour it may be almost as 

 complete as it was on the original upper surface. 



There, is however, a distinct polarity to the egg that is not influenced by its 

 position, for cleavage ultimately makes greater progress on that side of the egg 



FIG. 122. Cleavage stages, and yolk navel, h, of Limulus eggs. 



that is to become the neural surface, irrespective of whether it faces up or down. 

 On the fourth or fifth day, most of the eggs are covered with a single layered 

 blastoderm on one side, while the opposite side was still occupied by large yolk 

 spheres. 



In many cases, the blastoderm, spreads over the opposite side of the egg in an 

 advancing fold, the uncovered yolk either protruding from the opening, or lying 

 a little below the general surface. The uncovered area is usually circular, but it 

 may be quite irregular, and varies in size from an opening one-third the diameter 

 of the egg to that of a pin hole. (Fig. 126, h.) The yolk plug thus produced has 

 nothing to do with the formation of germ layers. It resembles the condition seen 

 in the early stages of certain ganoids, cyclostomes, and other fishes, but it has 

 no parallel, to my knowledge, in the invertebrates. 



On the eighth day, a single layered blastoderm covers the entire egg, enclosing 

 numerous nucleated yolk spheres. 



Comparison of Arachnids and Vertebrates. The cleavage in the arach- 



