EMBRYOLOay 



223 



expand, as in Fig. 178. One side becomes a thin layer of cells applied to 

 the digestive tract, while the other side lines the inside of the ectoderm. 



In telolecithal eml^ryos like that of the frog and the lamprey, the 

 endoderm is so thick that folded grooves like those of Amphioxiis could 

 hardly be formed. In such cases the mesoderm may be simply split off 

 from the outside of the endodorm; In Fig. 179, which represents a cross- 

 section of the gastrula of the lamprey, the slit (d) above the archenteron 

 represents the beginning of this 

 delamination. 



In the higher vertebrates the 

 formation of mesoderm is often a 

 very complicated or obscure process. 



vm 



Fig. 178. — Cross section of embryo of 

 Amphioxus, showing expansion of the meso- 

 derm between ectoderm and digestive tract. 

 c, ecehjin; dg, digestive tract; dm, dermatome; 

 ec, ectoderm; en, endoderm; mc, myoccele; 

 mt, mj'-otome; nd, notochord; nl, spinal cord; 

 som, somatic layer of mesoderm ; spl, splanch- 

 nic layer of mesoderm covering the digestive 

 tract; vm, ventral mesentery. 



Fig. 179. — Mesoderm formation by 

 delamination in the lamprey. The meso- 

 derm is simply split off the endoderm. 

 ar, archenteron; d, line of delamination; 

 ec, ectoderm; m, mesoderm; ng, neural 

 groove. (After Goette, courtesy of Leopold 

 Vosti.) 



ORGANOGENY IN THE VERTEBRATES 



The three layers of cells, ectoderm, endoderm, and mesoderm, are 

 often called the germ Imjers. They are so designated because certain 

 organs are derived from each one, so that the layers may be thought of as 

 containing the germs of those organs. Thus, f rom th e ectoderm come 

 the nervous system, important parts of the sense organs, the outeFlayer 

 of the skin, and some others; from the endoderm the lining of most of 

 the digestive tract, and of such out-growths of the digest iv(> tube as the 

 liver, pancreas, and lungsjlmdrfi-om the mesoderm the muscles, bones, 

 blood system, kidneys, etc. If all the facts were known it might be 

 possible to point out the specific cells, even in stages prior to gastrulation, 

 which will become certain organs of the embryo, just as in some animals 



