VDERMAL ORGANS. 





MORPHOLOGY OF THE ORGANS OF 

 VERTEBRATES. 



FROM the point where the four germ-layers are clearly dif- 

 ferentiated from each other, we have now to trace the various 

 derivatives of each ; but it must be kept in mind not only that 

 various organs are in the process of development at the same 

 time, while the necessities of treatment demand that they be 

 arranged in sequence, but that two or more layers not infre- 

 quently contribute to the same organ. In such cases the organ 

 is described in connection with that layer which is most promi- 

 nent or most important in its structure. In the following account 

 the stages of development are traced only with such detail as is 

 necessary for a clear interpretation of the adult structure. For 

 more extended accounts the student must go to the embryologi- 

 cal manuals and special memoirs. 



ENTODERMAL ORGANS. 



The differentiation of the entoderm by invagination has been 

 described (p. 5). By this process of gastrulation a layer of 

 entoderm cells comes to lie inside the other or ectoderm cells, 

 and by the closure of the blastopore (usually complete) it assumes 

 the form of a sac, the cavity of which is the archenteron. As 

 the embryo elongates, the sac forms an elongate tube. In the 

 middle line of its dorsal wall a cord of cells, lying between the 

 outgrowing ccelomic pouches (Fig. 7, ;/), becomes constricted 

 off from the rest, 1 and occupies a position between the other 

 entodermal structures and the nervous system (Figs. 7 and 20). 

 This rod is the notochord, the subsequent history of which is 

 given in connection with the skeleton. 



The rest of the entoderm, after the formation of the noto- 



1 In a few forms (e.g., Amblystoma) this cord is at first tubular ; later its lumen is lost. 



