THE DEVELOPMENT OF VERTEBRATA. 225 



later that connective, muscular, and nervous tissues come 

 to be formed from them. 



From EPIBLAST are developed the epidermis (not the 

 dermis) and the whole of the nervous system, including the 

 essential sensory part of all sense-organs. 



From MESOBLAST are developed all the connective tissues 

 (including dermis), all the muscles, blood-vessels and blood, 

 excretory organs and gonads in fact, the main mass of the 

 body. 



The HYPOBLAST gives rise to the epithelium of the ali- 

 mentary canal and of all glands that open into it ; but not 

 to any other part of the walls of the canal. The notochord 

 is also usually counted as derived from hypoblast, although 

 it appears before mesoblast and hypoblast are clearly 

 differentiated. 



These statements are true not only for Amphioxus, but 

 also for all Vertebrates, and indeed (with slight modifica- 

 tions) for the higher Invertebrates for all animals, in fact, 

 in which the three germinal layers are developed. It will 

 be convenient to indicate these three layers by distinct 

 shading in our diagrams, and therefore in many of them * 

 epiblast will be made black, mesoblast dotted, and hypoblast 

 lined. 



12. Later Development of Amphioxus. The neural 

 plate soon begins to roll itself up into a longitudinal tube, 

 and this becomes the spinal cord. The myocoelomic 

 pouches grow in size and push their way, as it were, 

 between the other layers until they have extended as far 

 as shown in fig. 115, A. They now become divided into a 

 dorsal and ventral portion, the fate of which is different 

 the dorsal portions retain their metamerism, their cavity 

 becomes obliterated by the growth of the walls, and they 

 give rise to the myomeres of the body. The ventral 

 portions lose their metamerism, their cavities becoming 

 connected into a single cavity the coelom from end to end 

 of the body, and the cavities of the right and left sides also 

 become joined ventral to the alimentary canal (fig. 115, B). 



Thus the mesoblast may be said to be divided into three 



* Figs 116, 120, 121, 123, and 130 to 137. 

 ZOOL. 1*> 



