The Development of Scyphomedusse. 225 



Claus (1883) was the next to study Aurelia aurita. He called 

 attention to the fact, known to early observers, that the eggs 

 pass from the ovaries into the gastric cavity and from there into 

 the folds of the manubrium where development takes place. 

 Haeckel was incorrect in thinking the first cleavage unequal and 

 especially in the differentiation of the two blastomeres, since, as 

 Claus stated, the first cleavage furrow passing through the animal 

 pole can not divide the egg into an animal and vegetal blastomere. 

 The second cleavage Claus found to be meridional and the third 

 equatorial. Up to this time cleavage was equal but thereafter 

 might be unequal and form a blastula with shorter cells at one 

 pole. As he found that the cleavage cavity, present as early as 

 the 8-cell stage, increased only slightly and consequently remained 

 small, he believed Haeckel was mistaken about the large cavity 

 and wide invagination. Because of the small size of the invagina- 

 tion the cells were crowded together and resembled very much the 

 appearance of the cell mass formed by ingression in Aequorea, 

 but he w^as able to show that a real invagination had taken place. 

 In Chrysaora Claus found essentially the same process of cleavage 

 except for the smaller egg, the development within the ovary and 

 the very large cleavage cavity. An ingrowth of cells from the 

 thicker portion of the blastula wall formed a solid plug but with 

 cells arranged in two rows, so that the process was quite close to 

 an invagination. As the archenteron grew the ectoderm also 

 extended, and for a long time there remained a space between the 

 ectoderm and entoderm. 



In Lucernaria Kowalevsky (1884) found a regular and equal 

 cleavage, the first and second furrows meridional, the third equa- 

 torial. No cleavage cavity was formed, segmentation resulting 

 in a solid mass of cells, an outer layer and an inner mass. The 

 latter had its origin from a delamination of the early cleavage 

 cells or, in some cases, from an actual immigration of entire cells. 

 From the inner cells the entoderm was formed, the outer row 

 being the ectoderm. Metschnikoff (1886) confirmed the absence 

 of invagination in Lucernaria, and also described the results of a 

 study of Nausithoe marginata and Pelagia noctiluca. In the 

 former, although the third and fourth cleavages were unequal, the 



