586 HAROLD HEATH, 



fourth quartette) aad possesses as regards position the character of 

 the other secondary stomatoblasts which arise somewhat later than 

 the posterior. There are however some important features which 

 distinguish this storaatoblast from those of the other three quadrants, 

 and I have reserved a consideration of its characteristics for the 

 section on the anachronism of cleavage. 



7, Second Cleavage of Third Quartette; Division of 

 Accessory Trochoblasts, 72 cells (Figs. 20, 21). 



The upper products of the third quartette are the next cells to 

 divide. In each the spindle is perfectly horizontal and the resulting 

 cleavage produces two cells of equal size. This is the first of a series 

 of cell divisions in which the spindles inaugurating the division lie 

 at right angles to those of the preceding cleavage (Fig. 21). 



The spindles form in the posterior quadrants and the cell divisions 

 occur slightly in advance of those of the anterior (Fig. 20) ^). 



The only division which the accessory trochoblasts undergo occurs 

 at this stage. The spindles arise simultaneously and placing them- 

 selves in a dexiotropic position divide each cell in two equal halves 

 (Fig. 21). The primary trochoblasts now number six in each quadrant, 

 and with the exception of an increased external surface they occupy 

 the same position as at their formation. 



IV. Formation of Mesolblast and Entoblast. 



1. Primary Mesoblast, 73 cells (Fig. 22). 



The mesoblast arises from the posterior macromere by a cleavage 

 in which the spindle is generally slightly leiotropic (Fig. 22), though 

 in many cases it assumes a perfectly radial position. Usually as a 

 result of the division by far the greater part of the substance of the 

 macromere forms the mesoblast, the remainder, D, being often scarcely 

 larger than the full formed mesoblast nucleus. In other cases the 

 macromere is a relatively long and slender cell extending for a con- 

 siderable distance into the egg. 



At the time when the mesoblast forms it becomes shifted in some 

 unaccountable way slightly to the right; the third quartette stomato- 

 blast 3c^ becomes somewhat smaller superficially and the outer border 



1) Fig. 20 is almost an exact copy of Kowalevski's fig. 14 which 

 represents the vegetative pole. Metcalf is therefore probably in error in 

 believing that this is a figure of the animal pole. 



