No. I.] THE EMBRYOLOGY OF CREPIDULA. 27 



each quadrant, except the left posterior one. These three cells 

 are derived from the advancing edge of ectoblast, and from them 

 the scattered mesoblast cells around the blastopore apparently 

 originate. The other three members of the fourth quartette 

 are purely entoblastic, and they form the lateral and ventral 

 walls of the mesenteron. The residue of the four macromeres 

 is entirely entoblastic, and after they have given rise to a fifth 

 quartette of large yolk cells they form the dorsal wall of the 

 mesenteron. 



Gastndation. — The gastrula is formed by epibole associated 

 with a flattening of the macromeres ; there is no invagination. 

 The blastopore closes near the middle of the ventral side, and 

 at this point the mouth soon afterward appears. 



The Ectoblastic Cross. — When the stage with forty-two cells 

 has been reached, there appears at the upper pole of the Qg^ a 

 cross of ectoblast cells ; the centre of the cross lies exactly 

 at the animal pole, while each of the arms lies between the 

 first and second cleavage planes. Later the whole cap of 

 ectoblast shifts position so that the arms of the cross lie 

 approximately over those cleavage furrows ; thus one arm 

 comes to be anterior, one posterior, one right, and one left. 

 In the further development all the arms lengthen, and all save 

 the posterior one divide longitudinally into two parallel rows 

 of cells. All the cells of the cross are derived from the first 

 quartette save the "tip," or terminal cell, of each arm, which 

 comes from the second quartette. A single ectoblast cell, 

 which is at one time the smallest in the &gg, but which after- 

 wards becomes the largest, lies in the angle between adjacent 

 arms of the cross. There is one of these in each quadrant, 

 and because of their position and shape they are called for the 

 present the "turret cells." In later stages at least two of them 

 contribute to the formation of the velum. 



Change of Axes. — During the later stages of cleavage and 

 throughout gastrulation, the whole of the ectoblast at the 

 upper pole moves gradually forward through an angle of about 

 90°, so that the centre of the cross, which originally lay at the 

 middle of the future dorsal region, comes to lie at the anterior 

 end of the long axis of the embryo. The entoblast seems to 



