A PRELIMINARY ACCOUNT OF THE CLEAVAGE 



OF ARENICOLA CRISTATA, WITH REMARKS 



ON THE MOSAIC THEORY. 



C. M. CHILD. 



I. CLEAVAGE. 



THE eggs in the jelly in which they were laid were fixed in 

 picro-acetic acid and preserved in alcohol. This treatment 

 renders the jelly perfectly soluble in distilled water, so that 

 the eggs can easily be freed from the jelly by allowing a mass 

 of it containing eggs to stand a few minutes in distilled water. 

 The jelly disappears, and the eggs sink to the bottom. They 

 are stained in dilute Delafield's haematoxylin, cleared and 

 examined in clove-oil. 



The individual egg is fairly well filled with small yolk 

 spheres, which are scattered throughout all parts of it and 

 are found in all the cells of the earlier stages. The cleavage 

 conforms closely to the so-called "spiral" or "oblique" type, 

 especially in its earlier stages. Later, as in the other forms 

 of this type, cleavages which follow entirely different laws 

 occur. 



Figs, i, 2, and 3 represent respectively the 2-, 4-, and 8-cell 

 stages from the upper pole. In Fig. i both cells are already 

 in division, and the spindles do not lie in the same plane, but 

 are inclined to each other, so that this division is a true oblique 

 segmentation. In Fig. 2 the second division has occurred, 

 producing one enormous cell (D) and three much smaller ones 

 (A, B, C). The large cell D is in contact with its opposite B, 

 at both poles of the egg, i.e., the two cross-furrows are parallel. 

 The cross-furrow at the lower pole is longer than the other . 

 and perfectly constant up to a late stage, so that it furnishes 

 an invaluable means of orientation. 



It lies at right angles to the future median plane of the 

 adult. The cell D is dorsal, B is ventral, and A and C are left 



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