i66 Papers from the Marine Biological Laboratory at Tortugas. 



Beneath the peripheral layer is a layer of closely crowded yolk spherules 

 in which the nuclei lie during the early and late cleavages : this yolk-rich 

 layer constitutes the principal part of all the cells of the blastnla and gastrula. 



The central area of the egg contains scattered yolk spherules within a 

 semi-fluid yolk or matrix. During the cleavage this matrix is poured 

 into the cleavage cavity, where it seems to serve as a kind of fluid yolk for 

 the nourishment of the surrounding cells. The central area of the egg is 

 thus the precursor of the cleaixige cavity and its contents. 



The view expressed in my preliminary note on the development of 

 Linerges (1906), that the 3 layers of the egg give rise to the ectoderm, the 

 endoderm, and the mesoglcea is not confirmed by further studv. Portions 

 of each of these 3 layers are found in all the cells of the blastula and gas- 

 trula, and consequently in both ectoderm and endoderm ; therefore these 

 substances are not organ-forming with respect to the germinal layers. Never- 

 theless, each of these substances, under normal conditions, produces a specific 

 part of the embryo. The fact that parts of the unsegmented egg or isolated 

 blastomeres may give rise to entire planulse may be due to the fact that such 

 parts or blastomeres contain the ooplasmic substances of all of these 3 layers, 

 owing to the concentric arrangement of these substances. It is not easy to 

 isolate these substances and observe the development of each, for although 

 they may be more or less completely separated by strong centrifugal force, 

 they do not, in most cases, undergo further development : and the more com- 

 pletely these substances are separated the less capable they are of develop- 

 ment. Nevertheless, as far as my experiments go they indicate that these 

 ooplasmic substances are not individuall}- totipotent. 



Mechanics of cell-division. The peculiar form of cell-division found 

 among ccelenterates has attracted much attention. Ziegler (1898. 1903), 

 Rhumbler (1899), and Fischel (1898, 1906) have dealt with this problem 

 in a comprehensive manner. In brief, Ziegler holds that cell-division is 

 brought about by the activity of the outer protoplasmic layer, the unilateral 

 constriction being due to a heaping up of this layer to form a " cleavage 

 head " at the animal pole ; this heaping up he regards as the result of the 

 action at a distance (" Fernwirkung ") of the centrosome. Rhumbler also 

 finds the cause of unilateral constriction in the heaping up of the peripheral 

 layer in the cleavage head, due to the astral rays, and in the increased 

 membrane formation in the plane of cleavage, due to the escaped nuclear 

 substances which lie in this plane. Fischel believes that the cleavage is 

 e.xplicable on the assumption that the astral rays are contractile threads, 

 and that the unilateral constriction is due chiefly to the peripheral position 

 of the nucleus and centro.some. 



In the case of Linerges a glance at figure 6, and the photomicrographs 38 

 and 42 shows that while the nuclei and centrosomes lie near the animal pole 

 of the egg they do not lie in the peripheral layer of protoplasm. Furthermore, 



