Embryology. 141 



other such forms in nature, the first locomotive and 

 transitory, the second fixed and permanent (Figs. 

 43, 44). 



Here, then, we leave the lower forms of Metazoa in 

 their condition of permanent gastrulae. They differ 

 from the transitory stage of other Metazoa only in 

 being enormously larger (owing to greatly further 

 growth, without any further development as to matters 

 of fundamental importance), and in having sundry 

 tentacles and other organs added later on to meet 

 their special requirements. The point to remember 

 is, that in all cases a gastrula is an open sac composed 

 of two layers of cells the outer layer being called the 

 ectoderm, and the inner the endoderm. They have 

 also been called the animal layer and the vegetative 

 layer, because it is the outer layer (ectoderm) that 

 gives rise to all the organs of sensation and move- 

 ment viz. the skin, the nervous system, and the 

 muscular system ; while it is the inner layer (endoderm) 

 that gives rise to all the organs of nutrition and 

 reproduction. It is desirable only further to explain 

 that gastrulation does not take place in all the Metazoa 

 after exactly the same plan. In different lines of 

 descent various and often considerable modifications 

 of the original and most simple plan have been intro- 

 duced ; but I will not burden the present exposition 

 by describing these modifications l . It is enough for us 

 that they always end in the formation of the two 

 primary layers of ectoderm and endoderm. 



The next stage of differentiation is common to all 

 the Metazoa, except those lowest forms which, as we 



1 The most extreme of them is that which is mentioned in the last 

 foot-note. 



