574 DEVELOPMENT OF THE LANCELET CHAP. 



it is necessary to select for our study an alecithal holo- 

 blastic egg, such as that of the lancelet. 



Before reading the rest of this chapter, however, you should once 

 more glance at the account of the development of the tadpole given on 

 pp. 198-210, in order to refresh your memory of some of the more 

 important facts and of the terms used. 



Development of the Lancelet. The oosperm of Am- 

 phipxus (Fig. 151, A) undergoes binary fission (B), each 

 of the two resulting cells dividing again into two (c, D). 

 This process is continued until a globular mass of cells or 

 blastomeres is produced by the repeated division of the 

 one cell which forms the starting point of the series. 

 Owing to there being rather more yolk at the lower than at 

 the upper pole, the lower cells are slightly larger than the 

 upper, so that the segmentation is not quite equal (E K). 

 The embryo has now arrived at the polyplast or morula 

 stage, and sections show that it is hollow, the blastomeres 

 being arranged in a single layer around a central segmenta- 

 tion-cavity (p. 200) : such a hollow polyplast is often known 

 as a blastula (K). The lower side of the blastula then 

 becomes tucked in, or invaginated, the result being that the 

 single-layered sphere is converted into a double-layered cup 

 (Fig. 152). This process can be sufficiently well imitated 

 by pushing in one pole of a hollow india-rubber ball with 

 the finger. The resulting embryonic stage is known as 

 the gastrula (c) : its cavity is the primitive enteron 

 or archenteron (p. 201), and is bounded by the invaginated 

 cells which now constitute the endoderm, the remaining cells 

 forming the outer wall of the gastrula being the ectoderm 

 (p. 202). The two layers are continuous at the aperture 

 of the cup, the gastrula-mouth or blastopore. Between the 



