760 PROCEEDINGS OF THE ACADEMY OF [DeC, 



tlie plane of fission, and at llie same distance from the surface'^of 

 the egg as the original nucleus. The second furrow normally starts 

 on the same point of the surface as the first, and again divides the 

 egg meridianally in a plane at right angles to the first. Sometimes 

 the second furrow starts irregularly, at a point around the egg 

 from the origin of the first furrow. One of the hemispheres is 

 thus divided before the other, as in fig. 7. Fifty minutes elapses 

 between the completion of the first and second furrows. Succes- 

 sive segmentations come in at intervals of forty -five to fifty minutes. 



AVilh the eight-cell stage rotation of the blastomeres occurs. The 

 four upper cells turn through an angle of 45° upon the lower ones, 

 so that they come to lie in the valleys between the lower ones, 

 instead of being superimposed upon them. Segmentation continues 

 until a hollow blastula (PI. XXXI, fig. 8) is produced, a layer of 

 thick cells surrounding a small cleavage cavity. The cells are of 

 uniform thickness, and their outer ends give rise to cilia which 

 drive the egg roimd and round by their motion within the mem- 

 brane, sometimes in one dii'ection, sometimes in the opposite. 



During this stage the formation of the endoderm takes place. The 

 inner ends of the blastomeres are delaminated, the process going 

 on at an equal rate on all sides, until a uniform layer of endoderm 

 cells lies within the ectodermal layer (PL XXXI, fig. 9). By 

 increase in size of these endodermal ""cells the cavity of the egg 

 comes to be entirely obliterated. During the subsequent life- 

 history of the larva no cavity exists within the body until after 

 several marked changes have taken place. 



The Planula. 



By the rupture of the egg membrane the nearly spherical 

 ciliated larva makes its escape, and starts upon the stage in which 

 it is a swimming planula (PI. XXXI, fig. 10). Its shape soon 

 changes, becoming narrower and longer at one pole than the other ; 

 this narrower pole is to be the future oral extremity of the larva. 

 The cilia serve to propel the planula in a slow rotating j)rogression 

 through the water, usually not far from the bottom. The larger 

 end is directed forward in swimming. The time at which the 

 planula appears is in the morning, about twelve hours after the 

 egg was fertilized. The length of the larva is now between . 1 

 and .15 mm. (PI. XXXI, fig. 10). This condition persists for a 



