XI MESODERM AND CCELOME 565 



the upper pole of a large blastodermic vesicle {bv], representing the yolk- 

 sac of a bird but containing a fluid instead of yolk' and being 

 surrounded by a layer of cells known as the Iropliohlast [ef). 



In Amphioxus alone amongst the triploblastic animals 

 deseribed in this book, does the mesoderm arise as a series 

 of enterocoelic pouches : it is usually at first solid, and may 

 be budded off from the endoderm, from the lip of the 

 blastopore or primitive groove at the junction of the ecto- 

 derm and endoderm, or both endoderm and mesoderm may 

 be differentiated at the same time from the lower layer- 

 cells or yolk-cells (e.g. frog) ; or, finally, it may arise in 

 all these ways {e.g. fowl, rabbit). The coelome is formed 

 by a split taking place in the mesoderm on either side 

 (Figs. 65, mes, and Fig. 147, msd,som, spl), the split gradually 

 extending with the extension of the mesoderm between 

 the ectoderm and endoderm. Thus the ccelome is formed, 

 not as an enterocoele, but as a schizoccek. 



In Vertebrates each mesoderm-band becomes differ- 

 entiated into a dorsal portion, the vertebral plate, which 

 soon loses its coelomic space, and a ventral portion, the 

 lateral plate, which is divided into parietal and visceral 

 layers by the coelome (Figs. 143 D and 147). The vertebral 

 plate undergoes metameric segmentation, becoming divided 

 into a row of squarish masses, the mesodermal segments or 

 protovertebrce {pr. v), from the dorsal portions of which 

 the muscular segments or myomeres are formed (p. 203), 

 and from their ventral portions the vertebral column, the 

 segmentation of which alternates with that of the myo- 

 meres. 



Development of the chief organs in the Craniata 



(compare pp. 201-210). — The nervous system, as well as 

 the essential parts of the sensory organs are, as we have 



