726 PROCEEDINGS OF THE ACADEMY OF [Oct., 



Dinophilus, for example, the members of the first quartette when' first 

 formed lie in the furrows between the macromeres ; at the next cleav- 

 age they are shifted in a sinistral direction, so that they lie precisely 

 over their parent macromeres (cf. figs. 8 and 10), and this relative 

 position is retained up to the formation of the fifth quartette. In 

 Amphitrite (Mead, 1897) and Arenicola (Child, 1900) this sinistral 

 totation does not bring the micromeres so precisely over the corres- 

 ponding macromeres. 



In considering the relation of the cleavage plane with the embryonic 

 axis, the micromeres are of far more importance than the macromeres. 

 An examination of the figures of the later cleavages will show that the 

 planes of the first and second cleavages as traced over the entire embryo 

 do not coincide with its transverse and sagittal planes, but that here 

 the rule laid down by Lillie (1895) holds good, viz.: "The members of 

 the odd generations of ectomeres, as well as the entomeres, are 

 distributed two each, right and left of the middle line; those of the 

 even generations are placed anterior, posterior, right and left." 



The shifting of the axis, which is largely concerned in bringing about 

 the closure of the blastopore, has been already described in chapter V. 

 It was there spoken of as a forward movement of the first quartette, 

 or rather of the ectoderm of the anterior half of the embryo. This 

 shifting, I think, might better be looked upon as a shifting backward 

 of the ectoderm of the posterior half of the embryo, considering the 

 animal pole as the fixed point. In the first case the blastopore would 

 be closed from in front backward; in the latter from behind forward, 

 which brings it into line with what we know of other forms. In either 

 case the egg axis is bent through an angle of nearly 90 degrees, but not 

 quite, since the animal pole is, as Wilson (1892) showed to be the case 

 in Nereis, probably slightly dorsal to the anterior pole of the embryo. 

 This shifting of areas during the process of gastrulation is almost pre- 

 cisely similar to that which occurs in Crepidula, as described by Conk- 

 lin (1897). There is in that form a bending of the egg axis through 

 90 degrees, and also caused by rapid growth of the cells of the D quad- 

 rant, which pushes forward the ectoderm of the aboral surface over the 

 entoderm, the vegetative pole being apparently the fixed point. This 

 condition is attributed by Conklin to the accinnulation of yolk in the 

 entoderm cells, and doubtless this is the cause of the similar relations 

 existing between entoderm and ectoderm in Dinophilus, the solid 

 yolk-laden mass of entoderm cells altering their form and relative 

 positions but little, while the ectoderm is shifted over them. 



