BIGELOW: EARLY DEVELOPMENT OF LEPAS. 101 



ered ventral portion (Fig. 43) are mes-ectoblasts, and the remaining 

 eleven dorsally-lyiug cells contain only ectoblast. 



Figures 42-46 (Plate 5), and 107-113 (Plates 11, 12), show the 

 positions of the cells in the sixteen-cell stage, regarding which it will be 

 sufficient to call attention to their bilateral arrangement. All the cells 

 of the eight-cell stage, with the exception of the cell b*-^, which lies at 

 the vegetative pole (Fig. 40), divide so that their daughter cells both 

 lie either on the right or on the left of the median plane of the embryo. 

 The exceptional cell, 6*-\ divides in a plane parallel to the plane of the 

 preceding cleavage, and, consequently, the daughter cells {b^-'^ and b^-'^) 

 are not separated by a plane coinciding with the median plane of the 

 embryo (see Figs. 40 and 43). 



The regular and definite arrangement of the cells represented in the 

 figures of the sixteen-cell stage is quite noticeable. This first suggested 

 to me that the arrangement had arisen from an equally definite one in 

 the earlier stages. Figures of a similar stage accompany the accounts 

 of other investigators, who seem to have observed a constant arrange- 

 ment of the cells in this stage. 



At the sixteen-cell stage the " protoplasmic " cells have become ex- 

 tended far over the yolk-cell (compare Plate 5, Fig. 40 with Fig. 45, and 

 Plates, Fig. 66 with Fig. 68). This extension is due in part to the 

 addition of a new cell (the primary mesoblast) from the yolk-cell, but 

 more especially to the spreading of the blastoderm, which is caused by 

 division of the derivatives of the three micromeres {ab% c*, d*'^). 



The blastopore is marked by that portion of the entoblast cell (d^'^), 

 which is still exposed to the exterior (Figs. 45, 46, 68), and it is widely 

 open. Eggs with a relatively small amount of yolk have the blastopore 

 more nearly closed ; but, as will be shown later, the number and order 

 of cleavages are constant whether an egg contains a large or a small 

 amount of yolk. 



Summary of the Fourth Cleavage. 



A sixteen-cell stage is regularly formed with cells of partictilar origins 

 occupying definite and constant positions in relation to other cells. 



The derivatives of the three micromeres {ah'^, c^, d*-^) divide synchron- 

 ously. The yolk-cell d*-^ (mes-entoblast) is delayed in cleavage. 



The primary mesoblast (d^-^) is separated from the yolk-cell c?*-^, 

 which is now entoblast. 



The blastoderm is greatly extended during the fourth cleavage. 



