DEVELOPMENT OF EGGS IN MANY-CELLED ANIMALS 345 



liver, which in the Lancelet is a simple pouch. And the endoderm 

 also gives rise to the elastic rod or notochord, which represents the 

 backbone in this particular animal, and is replaced partly or en- 

 tirely by the backbone in higher Vertebrates. The origin of this 

 rod is indicated in fig. 864. The wall of the upper part of the 

 digestive cavity is seen to be grooved in B, just between the two 

 mesodermic pouches. In c the groove is better marked, and in d 

 a strip of the wall of the digestive cavity has been pinched off to 

 form a rod, of which the section appears oval. 2. The outermost 

 layer {ectoderm) naturally gives rise to the epidermis, or external 

 part of the skin. From it the nervous system and the essential 

 parts of the sense organs also take origin. The development of 



C D 



,-i*nTHO.IITTi^ 



A" V t ,, -N N ■> C 



- ""S ■ 1» " ' ' >3 I 



y 



Fig. 864. — Cross-sections through older Embryos of Lancelet [A m//izo,rits), enlarged. IMesoderm darkly 

 shaded. The central space is the digestive cavity. See text. 



the hollow dorsal tube which constitutes the central part of the 

 nervous system is seen in fig. 864. A central strip of ectoderm on 

 the upper side of the body is thickened into a sort of plate, which 

 sinks into the body and then folds up into a tube. Examination 

 of the upper parts of a, b, c, and D will make this clear, remem- 

 bering, of course, that only thin cross-sections of this strip are 

 shown. 3. The middle layer (mesoderjn) is the parent of all the 

 remaining organs, as, for example, those concerned with circulation 

 and development of eggs. The muscular system and the muscle 

 in the wall of the digestive tube are also products of this layer, as 

 are the deeper part of the skin (dermis) and all the supporting 

 fibrous tissue found throughout the body. 



The fate of the three embryonic layers, as just summarized, is 

 practically the same in all three-layered animals, from backboned 

 forms down to Worms and Echinoderms. 



Influence of Food-Yolk on Development (fig. 865). — The egg- 

 cells of different animals vary greatly in size, according to the 

 amount of nutritive matter, or food-yolk, which is stored up in 



Vol. hi. 85 



