224 



EMBRYOLOGY 



When the development of the worm has progressed as 

 far as this, it breaks through the amnion and pilidium and 

 swims about free in the water by means of its covering of 

 cilia. At this stage it lacks the anus, which arises only 

 later. In some cases eye-spots are present ; in others they 

 are absent. 



II.— Development after the Type of Desor. 



For the more intimate knowledge of the mode of develop- 

 ment in the type known as that of Desor, we are chiefly 

 indebted to J. Barrois (No. 1). Recently Hubrecht (Nos. 9 

 to 11) has reinvestigated the subject. 



Here also, as in the development of the pilidium, an 

 invagination gastrula arises, which at first is radial, subse- 



A 



Fig. 105. — A to C, formation 

 of the somatic plates by in- 

 vagination in Lineus ohscurus 

 (after J. Bareois). 



Fig. 106.— Section of an em- 

 bryo of Lineus ohscurus (after 

 Hubrecht). D, intestine; if, 

 mouth, which, however, like 

 the oesophagus, is closed by 

 cells ; Mes, mesenchyma cells ; 

 S, discs which subsequently 

 form the ectoderm of the worm. 



quently bilaterally symmetrical. According to Hubrecht, 

 cells (the mesenchyma cells) are said to migrate into the 

 blastocoele from both ectoderm and entoderm (Fig. 106). 

 On the ventral surface of the ectoderm Barrois found a pair 

 of invaginations in front of the mouth and another behind 

 it. He saw that these invaginations were closed by the 

 growth of the ectoderm over them, and that finally their 

 floor became separated from the rest of the ectoderm (Fig. 



