MONOGEAPH OF THE EXISTING CRINOIDS. 437 



the circumference of the embryo, from the borders of which the invagination of the 

 .entoderm takes place. 



The monaxial bhistula has thus become a bilateral gastrula. 



The inner cavity is, as in the preceding and succeeding stages, filled with 

 an albuminous substance. 



Both the ectoderm and entoderm cells are more or less prismatic in shape, 

 and in both series the cells are arranged in a single layer. 



The nuclei of the ectoderm cells lie in their outer halves, often very near 

 the periphery. In the entoderm cells they occur mostly in the middle. In the 

 cells which, as a result of the abrupt folding, are more or less pyramidal in shape, 

 they lie in the broad cell end, whether this is toward the lumen of the primitive 

 gut or toward the primary body cavity. 



Among the prismatic cells there are in the ectoderm a number of globular cells 

 situated exteriorly and excluded from the primitive body cavity by the arching 

 together of the neighboring prismatic cells beneath them. These are ectoderm cells 

 undergoing division. 



The number of cells in the blastula continues to increase. The elements of the 

 single-layered ectoderm become relatively higher and narrower, while those of the 

 entoderm appear by contrast more rounded. The lumen bounded by the latter 

 always retains the form of an elongate very narrow fissure. 



While at the time of the first formation of the blastopore the gastrula was 

 bilaterally symmetrical, this is no longer true, for the invagination of entoderm 

 cells takes place with slightly greater rapidity on one side than on the other, and 

 the blastopore, now more or less constricted in the middle, where the invagination 

 is deepest, is placed somewhat diagonally. There is considerable variation in the 

 form of the blastopore as well as in that of the primitive gut. 



During the entire course of the first night the embryo continues in the gastrula 

 stage. The cells increase in number, and mesenchyme formation begins in the 

 entoderm. 



The blastopore now has the form of a narrow crescent the concave side of 

 which corresponds, within the embryo, to the side toward which the entire mass of 

 invaginated cells bends over. That is to say, within the gastrula there is a more 

 extensive free body cavity on the side of the primitive gut corresponding to the 

 convexity of the blastopore than on the other. 



As in the preceding stage, there is considerable variability in the form of the 

 blastopore and of the primitive gut. 



It is possible with a reasonable degree of correctness to indicate the orienta- 

 tion of the 16-hour gastrula in terms of the free-swimming larva. The side toward 

 which the primitive gut leans will become the ventral. As the blastopore is 

 posterior the right and left sides are also determined. 



In some cases embryos of older stages are found in which the anterior 

 (innermost) end of the primitive gut is turned toward the dorsal instead of 

 toward the ventral side, later reversing its position. It is thus evident that the 

 side toward which the gut leans at this stage is not necessarily the ventral side. 



