GERM LAYERS. 71 



The mesodermic somites meanwhile extend their edges ventrally between 

 the ectoderm and the entoderm until they meet and fuse in the midventral line 

 (Figs. 56, d and 59). A transverse constriction next appears which cuts off the 

 ventral extension. The latter is known as the lateral plate, w r hile the remaining 

 dorsal part is still designated the primitive segment. (Compare Fig. 56, d } with 



Fig. 59)- 



The primitive segments retain their segmental character. The lateral 

 plates, on the other hand, do not retain their segmented condition but fuse, their 

 cavities uniting to form the primitive body cavity or ccelom, which is the anlage 

 of the large serous cavities of the adult. The outer part of the lateral plate or 



Neural tube 



^ Notochord 



Epidermis (ectoderm) 



^^JS/^>^t^Q"\ m^^^ 



' Primitive segment 



Muscle plate 

 Cutis plate 

 Myocoel 



Splanchnocoel 



Parietal mesoderm ~) 



Mat. plate 

 Entoderm -W^^^H^S/^ Visceral mesoderm J 



Ventral Subintestinal 

 mesentery vein 



FIG. 59. Diagram to show differentiation of primitive segment into muscle plate (myotome) and 

 cutis plate and relation of myocoel and splanchnocoel. Bonnet. Compare with Fig. 56, d. 



parietal mesoderm, with the adjacent ectoderm, forms the somatopleure (Fig. 59). 

 The inner layer of the lateral plate, the visceral mesoderm, with the adjacent 

 entoderm, forms the splanchnopleure (Fig. 59). 



At the caudal end of the embryo, just in front of the neurenteric canal, there 

 exists at this stage an area where the germ layers have not become differentiated 

 to form special structures. In this area, cell proliferation is especially active and 

 from it cells are derived for the completion of the neutral tube, chorda, somites, 

 intestine, etc. By this means the growth of the embryo in length is provided 

 for (Figs. 57 and 58). 



The Amphioxus embryo at this stage thus consists of: 



1. Ectoderm. Surface ectoderm and neural tube. 



2. Mesoderm. Somites; parietal mesoderm and visceral mesoderm 



enclosing the ccelom. 



3. Entoderm. Chorda and wall of primitive intestine. 



