126 BULLETIN OF THE 



The whole process of cleavage occupies about ten hours.* A rota- 

 tion of the spheres of segmentation according to A. Agassiz occurs in 

 Strongylocentrotus. This was not observed in Echinarachnius. Through- 

 out all the changes the egg is enclosed in the capsule, cap, which has 

 been mentioned in the unsegmented egg. 



Shortly after the end of the first half day after fecundation, the blas- 

 tomeres arrange themselves superficially about the segmentation cavity, 

 forming a hollow sphere, which is the blastosphere, PI. II. fig. 14. Mi- 

 nute cilia, which ai-e long and fine, appear over its surface, and the egg 

 begins to rotate and fret against the sides of the envelope or egg capsule, 

 which closes it in. There is no solid morula stage; but a true blastula 

 is immediately formed. At this time a thickening of the blastoderm at 

 one pole takes place, the outline becomes more pyriform, PI. II. fig. 15, 

 and at the truncated pole a collection of pigment of deeper color than 

 in the remainder of the ovum congregates. This increase in thickness 

 of the cells at one pole is indicative of the formation of a gastrula 

 mouth at that pole. Immediately after the thickening of the blasto- 

 derm an infolding begins to take place at this pole, PI. II. fig. 1 6. By 

 this infolding, ga, the layer of cells which form the walls of the cavity, 

 or the blastoderm, are infolded, and form the hypoblastic layer, or walls 

 of a gastrula stomach. The infolding is at first very slight, but the 

 increasing age of the embryo carries the walls deeper and deeper into 

 the cavity. 



With the first indication of an ingrowth of the gastrula stomach, or 

 archeuteron, we find budding off into the segmentation cavity certain 

 cells, a cl, which from their form, position, and other characters, are 

 called the amoeboid or mesoblastic cells. They give rise to important 

 structures, which later appear in the embryo, between epiblast and 

 hypoblast, and which belong to the middle layer or mesoblast. Prouho t 

 finds in Dorocidaris that these cells are not all the same. When his 

 paper came into my hands it was too late to verify in Echinarachnius 

 what he finds in Dorocidaris. At the time my observations were made 

 all the so-called amoeboid or mesoblastic cells were regarded as the same 

 in character, and although I supposed that they did not all form the 

 same structures, their differentiation in form was thought to take place 

 much later than in the gastrula stage. These cells form on each side of 

 an axis, passing through the gastrula mouth or primitive infolding. 

 Their bilateral arrangement was not as marked as in Ophiopholis. They 



* Kate of growth in water of unrecorded temperature. 

 t Comp. Rendus, ci. pp. 386-388. 



