26 EMBRYOLOGY 



total unequal cleavage, bj means of which there arises an 

 embryo consisting for the greater part of large blastomeres 

 rich in yolk (entodermic portion, Eig. 8 En), and presenting 

 only at its npper pole a cap of small blastomeres containing 

 little yolk (ectodermic part. Fig. 8 Ec). Since the cap of 

 ectoderm cells gradually grows around the entire embryo, 

 there is formed in this way a kind of epibolic gastrula. At 

 an early period there appears in the entodermic mass an 

 eccentrically placed irregular cavity, which can be referred 

 neither to a cleavage cavity nor to an archenteron, and is 

 known as the entodermic cavity. The chief axis of the 

 embryo is recognized by the eccentric position of this cavity, 

 for it always lies close to the apical pole (the subsequent 

 anterior pole). The embryo, which at first is comparable to 

 a plano-convex lens, now elongates in the direction of the 

 chief axis, and is covered on the surface with a coat of 

 cilia. The swarming larva (Fig. 9) is generally ovate 

 in form, and in floating in the water has the broadened 

 anterior end of the body, in which the spacious entodermic 

 cavity is situated, always directed upwards. The larva 

 possesses a superficial, unilaminar, flagellate epithelium, all 

 the cells of which present the same character. Within the 

 posterior half of the body is found a solid entodermic 

 mass, which, in the course of the further development, takes 

 on the character of embryonic connective tissue. The cells 

 lying in the vicinity of the cavity are flattened, and form a 

 reticulated layer of amoeboid elements. A similar layer of 

 flattened cells is found at the surface of the solid entodermic 

 core, where it is in close contact with the ectoderm. At an 

 early period spicules are developed in certain cells of this 

 core. 



The larva attaches itself by the apical pole, the layer 

 covering the entodermic cavity being split open, and the 

 edges of the breach thus formed bending outward. In this 

 way the cells of the entodermic layer come in contact with 

 the support, to which they adhere by means of pseudopodia- 

 like processes. In the majority of cases the larva after this 

 first attachment bends over in order thus to attach itself by 

 a broader surface. According to GtOette, there ensues the 



