DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



Ill 



(4) the remaining part, which, since it is destined to form the bounding 

 wall of the subsequent intestine (dh), is to be designated as permanent 

 entoderm (Darmclriisenblatt) (ik). 



The succeeding processes of development have as their objective 

 point the detachment from one another, by means of constriction and 

 fusion, of the parts which are still in continuity, and the formation 

 of discrete cavities. The processes of constriction begin at the 

 anterior end of the embryo, and progress thence to the blastopore 

 (figs. 70 and 71). At first the body-sacs become deeper (fig. 70 Ih), 



Ik 



ik 



dh 



Ik -'.JM. 



dh 



Fig. 71. 



Fig. 7-2. 



Fig. 71. Cross section through an Amphioxus embryo with five well-developed primitive seg- 

 ments, after HATSCHEK. 



ale, Outer, ik, inner, ink, middle germ-layer; nip, medullary plate; ch, chorda; dli, intestinal 

 cavity ; Ih, body-cavity. 



Fig. 72. Cross section through the middle of the body of an Amphioxus embryo with eleven 



primitive segments, after HATSCHEK. 

 ,', Neural tube; us, primitive segment. For the meaning of the other letters see Fig. 71. 



and then lose their connection with the main cavity (dh) by the close 

 apposition of the cells which surround the entrances to them (fig. 71). 

 By. this process the margin of the secondary entoderm (ik} comes to 

 abut directly on the margin of the chordal fundament (ch). The 

 latter has meanwhile also undergone changes ; the plate-like funda- 

 ment has become so curved by the elevation of its lateral margins, 

 that there has arisen a deep chordal groove, which is open along its 

 ventral side. Subsequently the lateral walls of the groove come into 

 close contact, and are thereby converted into a solid rod of cells, which 

 temporarily shares in the closure of the roof of the secondary intestine, 

 and appears as a ridge-like thickening of the latter. Then the cell- 

 rod (ch) becomes detached (fig. 72) from the wall of the intestine ; the 

 latter now, for the first time, becomes completely closed in the form 

 of a tube. To effect this the margins of the entoderm, indicated in 



