452 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 



but since the work of Boveri, Hertwig brothers, Roux, Driesch, Wilson, 

 Morgan, Loeb, and others on the fragments of eggs, the development 

 of embryos, abnormal and normal, from portions of eggs is a question 

 no longer to be doubted. 



FORMATION OF THE ECTODERM. 



In the development of the egg of Turritopsis the germinal layers are 

 not differentiated by the process of epibole, delamination, or cellular 

 ingression. During segmentation the blastomeres do not separate 

 and arrange themselves around a segmentation cavity which later is 

 transformed into a blastocoele. Thus instead of having formed a 

 coeloblastula, we find that cleavage results in the formation of a solid 

 oval embryo destitute of a blastocoele, which may be called a morula 

 stage. The cells of the segmenting egg are all alike in structure and 

 nearly equal in size; so that they are not distinguishable into primitive 

 ectoderm and primitive endoderm, which is the case in forms where a 

 definite delamination takes place, as is so beautifully shown in Liriope 

 and Geryonia, and in species where cellular ingression occurs as in 

 Stomotoca and Clytia for example. Figures 34 (pi. 33) to 39 (pi. 34) 

 illustrate the uniformity of the cells, and the solid character of the egg 

 during segmentation. In figure 36 (pi. 34) a space exists between 

 the blastomeres near one end of the egg, but this is not to be regarded 

 as a true cleavage cavity. The next figure shows three of these false 

 cleavage cavities. They occur only occasionally. As stated before 

 most of the eggs are entirely solid. 



About the time the irregular mass of segmenting blastomeres is 

 metamorphosed into the oval embryo, the cell boundaries are lost for 

 a short time and a syncytium is formed. This syncytial structure is 

 crowded with yolk granules and nuclei are scattered throughout the 

 protoplasm. The nuclei soon become more numerous near the peri- 

 phery; and then cell walls begin to appear as shown in plate 34, 

 figure 47. These cells are to become the ectoderm, which is soon 

 separated from the inner structureless mass by the development of 

 the mesogloea. Now the ectoderm forms a distinct layer, composed 

 of columnar cells all of which are at first similar in structure and lie 

 parallel to each other as shown in figure 48 '(?! 34). The differ- 

 entiation of the ectoderm cells takes place later. 



