306 EMBRYOLOGY 



into an ingrowth of cells (Fig. 144 A). As in the later stages 

 of the epibolic gastrula of Bonellia, here also the entoderm 

 consists of a solid mass with an outer differentiated cell-lajer 

 and an inner yolk-mass (Fig. 144 A). 



2. Larval Form and Metamorphosis of Echiurus 

 and Thalassema. 



The Echiuridas possess free-swimming larvfe, which exhibit 

 Tnore or less clearly the form of the Trochophore. The 

 development of the gastrula into the Trochophore takes place 

 in Thalassema by the appearance of a thickening of the ecto- 

 derm at the upper pole, the fundament of the apical plate. 

 This place becomes covered with a tuft of cilia in the same 

 way as in the larvae of the Polychteta. At an early stage 

 cilia make their appearance on the outer surface of the 

 embryo, and these are said to traverse, as in Eupomatus, the 

 egg-membrane, so that the latter would become the cuticula 

 of the larva (CoNx). Even during the gastrula stage a circle 

 of long cilia makes its appearance at the equatorial circum- 

 ference of the larva (Fig. 144 A). Below it lies the blasto- 

 pore. The cavity of the intestine arises as the result of the 

 absorption of the central yolk-mass by the rapidly multiply- 

 ing entoderm cells. It becomes connected with the outside 

 world by means of the mouth, which is formed at the place of 

 the blastopore. By the outgrowth of the hinder part of the 

 larva the mouth comes to lie more on one side (on the future 

 ventral surface) immediately under the band of cilia. The 

 latter is then differentiated into a row of cilia lying in front 

 of the moutli and one lying behind it (Fig. 144 B). In addi- 

 tion a ciliationmakes its appearance in the middle line of the 

 ventral surface. Tiie intestine grows so much in length that 

 it lies in loops. This is particularly true of the anterior part. 

 The terminal portion fuses at the hind end of the larva with 

 the ectoderm, thereby giving rise to the anus (Fig. 144 B). 

 Fore- and hind-guts, according to Conn, are entodermal 

 formations (?). Mesenchymatous muscle-cells extend be- 

 tween the ectoderm and entoderm of the larva, and at the 

 hind end lie two band-like cell complexes, the mesodermal 



