191 



pore is closed when the adult state is attained. Dorsally 

 at each side the hypoblast or endoderm (wall of the arch- 

 enteron) is pushed out, thus forming two longitudinal 

 folds (mesoblast pouches) ; these become constricted into 

 a succession of segments, and in that way, on each side, 

 a series of mesoblastic somites is formed (two pairs at this 

 stage, the others later), and the mesoderm is thus established. 

 (See Mesoderm, page 15, Part I.) 



Each cell of the ectoderm has a flagellum ; by means 

 of the flagella, the newly-hatched embryo swims. 



Give an Account of the later Development of the Embryo. 



The dorso-median endoderm (roof of archenteron) 

 develops a longitudinal ridge, which separates off and 

 forms a solid rod of cells between the archenteron and the 

 medullary tube ; this is the notochord. 



The mesoblastic somites extend down the sides of the 

 archenteron, between it and the ectoderm. The cells of 

 the somite walls next the notochord become elongated 

 muscle cells, and form the first <^ shaped myotomes (14 

 or 15). Anteriorly the mouth, a large oval opening on the 

 left side, is developed ; and also the first gill-slit, a small 

 opening on the ventral surface which is soon extended on 

 the right side. Later the anus is formed, near the hind end 

 and to the left of the middle line. 



The anterior part of the body is extended ; and there is 

 a corresponding extension forwards of the notochord. The 

 embryo lengthens out, becomes thin, and is tapered at 

 both ends. The tail is fringed by a large vertical fin. 

 The gut shows an anterior enlargement (pharynx) and a 

 long intestine. The mouth, anus, and first gill-slit having 

 been formed, the thirty - six - hours' embryo is now a. 

 larva. 



Mention the chief Developmental Features of the Larval 

 Stage of Amphioxus. 



The mesoderm segments (mesoblastic somites) are de- 

 tached from the archenteron, which has now a wall of definite 

 endoderm and becomes the enteron or gut. Additional 

 segments are successively developed (from the original 

 ones). The portions of the segment walls which adjoin 

 the ectoderm consist of somatic mesoderm ; those adjacent 

 to the spinal cord, notochord, and gut, consist of splanchnic 

 mesoderm. Each segment is divided into two portions, 

 a dorsal myotome (proto vertebra) and a ventral lateral plate. 

 The cavity of the myotome is the myocoele ; from its- 



