304 ALLEN. [VOL. I. 



and seems to be governed by no single law. In some cases 

 definite cell walls were found in the earlier stages, as in Fig. 2, 

 where four cells had been formed, one of which contained two 

 nuclei. In this we have only the stage next to the one last 

 described, but in that there were no segmentation planes at all. 

 In still later stages the development is quite as irregular. Fig. 5 

 shows a section in which six nuclei were visible, and other sec- 

 tions through the same egg contained several others, some of 

 which were in the process of division, but no cell walls had 

 been formed In Figs. 6 and 7 we have sections through 

 much older eggs, but the same indefiniteness of structure pre- 

 vails. From the foregoing illustrations it will be seen at once 

 that there is little uniformity in the early development of the 

 eggs of Parypha, either as to size of the cells formed or the num- 

 ber of nuclei that appear previous to the formation of the cell 

 walls. Segmentation does, however, begin at one pole, and the 

 greater part of the egg is for a time unsegmented. In no case 

 did I find the egg divided into two equal parts, as Dr. Hargitt 

 has sometimes observed in Pennaria eggs. PI. Ill, Fig. 3, 

 represents conditions similar to what is constantly met with in 

 eggs of Pennaria. In total segmentation the ovum consists of 

 a solid sphere of cells of more or less uniform size but with 

 irregular outlines. They are very reticular in structure, and 

 large vacuoles are numerous. 



Formation of tlie Ectoderm. 



Following the complete segmentation, the first indication of 

 a differentiation into ectoderm was observed in an increased 

 amount of cytoplasm in the outer layer of cells. These cells then 

 divide radially, forming narrow cells, as shown in text Fig. i. 

 The two mitotic figures lay in adjacent cells, as shown in the 

 drawing. In the next stage observed the ectoderm appeared 

 to consist of two layers of cells much smaller than those of the 

 endoderm, and distinguished from them by the greater density 

 of protoplasm. The two layers appeared in this case to be dove- 

 tailed into each other, as shown in Fig. 2 of the text. In the 

 fully formed ectoderm the cells are very elongate and somewhat 



