ECHINODERMS 8l 



During the early cleavages, a space is gradually formed between the 

 cells in the centre of the mass. This enlarges as the cleavage progresses, and 

 fairly soon after the last stage mentioned above, all the cells cohere to- 

 gether to form a smooth hollow spherical ball with this space in the centre. 

 This ball is the blastula, and the central space is the hlastocoel or blastula 

 cavity. The simple spherical shape is the most 'typical' form the blastula 

 can take; all the many variations which we shall find in other groups can 

 be regarded as modifications of it. In the echinoderms there are few 

 special features to notice; but one may remark that the surface of the 

 blastula soon becomes ciliated, and the tiny embryo escapes from its 

 membranes, and begins an independent Hfe soon after the cilia develop. 

 After a short time, it grows a bunch of special long cilia (the apical tuft) 

 at its animal pole. 



Soon after the apical tuft forms, the blastula loses its strictly spherical 

 shape and begins to flatten at the vegetative pole, diametrically opposite 

 the tuft. This is the first sign of gastrulation, which, in essentials, consists 

 in the folding inwards of the vegetative part of the blastula to form a 

 pocket pressed into the cavity of the blastocoel. This pocket is the first 

 rudiment of the gut, and is usually known as the 'primitive gut' or 

 archenteron. Its walls make up the endoderm, the innermost of the three 

 fundamental layers out of which the embryo is built. The opening by 

 which the gut communicates with the exterior is the blastopore. Mean- 

 while, the rest of the surface of the blastula, which is not pressed inwards, 

 forms the outermost layer or ectoderm. Between these, as we have seen, 

 there should be a third or middle layer, the mesoderm. The main way in 

 which the early development of the echinoderm differs from the general 

 scheme which applies to vertebrates is that here the mesoderm originally 

 lies, not between the ectoderm and endoderm, but right at the vegetative 

 pole. It is formed from the micromeres and the material just above them. 



experiments; white is an-i, fine dots an-2, coarse dots veg-i, circles veg-2, 

 and black the micromeres. Figure^ shows a section at the blastula stage, at 

 which time ciha develop on the surface; h, beginning of gastrulation and 

 formation of apical tuft of long ciha; /, the micromeres come free into the 

 blastocoel to form the primary mesoderm p.tn.;j, appearance of primitive 

 gut, with blastopore hi; from the tip of this secondary mesoderm [s.m) is 

 given off, the general ciliation is not shown in this or later drawings ; k, hori- 

 - zontal section through same stage as;; /, flattening of ventral side, the 

 skeleton begins to be laid down by the primary mesoderm; »i, n, views of 

 young pluteus larva from the original vegetative pole and from the side, 

 showing the development of 'arms' from the comers of the ventral surface, 

 and the junction of the tip of the primitive gut with the ventral ectoderm 

 to form the mouth. The dotted hne in n shows how the original animal 

 vegetative axis has become bent. (After Horstadius I939-) 



