EXISTING ORGANISMS— EMBRYOLOGY 51 



vertebrates. Explanation is inadequate to give an idea of this 

 resemblance, which is shown in Fig. 31. The structures which 

 have been described show that this resemblance is not merely 

 superficial, but is based on fundamental structure. 



Resemblance of Embryos to Adults of Lower Forms 



The resemblance of very early stages of vertebrate develop- 

 ment to the lowest animals has already been brought out. All 

 have their origin in a single cell, the fertilized ovum, morphologi- 

 cally equivalent to the single cell which makes up the entire body 

 of a protozoon. The blastula is quite similar to such colonial 

 protozoa as Volvox (Fig. 32), although we must recognize that 

 there is some differentiation in the cells of the blastula which is not 

 present in the cells of a Volvox colony. The simple gastrula has 

 been compared with coelonterates, such as Hydra (Fig. 33) and the 

 modifications of gastrulation brought about by the accumulation 

 of yolk in the ovum result in structures which can readily be 

 homologized with those of simpler forms. Beyond this point we 

 must seek reseml)lance of vertebrate with vertebrate as their 

 structures appear. 



The Notochord. Immediately after gastrulation a structure 

 called the notochord appears in all chordates (Fig. 34). It is a 

 rod of tissue whose origin is similar to that of the mesoderm al- 

 though their development is not directly associated. The noto- 

 chord extends through the body longitudinally between the 

 neural tube and the alimentary tract, and just ventral to the 

 former so that it corresponds in position to the main axis of 

 the spinal column. In the cyclostomes it persists throughout life; 

 in the remaining vertebrate classes it is well developed onl}^ in the 

 embryo and is either vestigial or absent in the adult. One striking 

 evidence of relationship is the similarity of development of the 

 notochord in reptiles and mammals. Even the birds, which are 

 in general rather closely related to the reptiles, show a marked 

 modification of the process. As the bones of the axial skeleton 

 develop, centra of the vertebrae replace the notochord at least 

 in part. 



The Skeleton. Lower Fishes. If we examine the most primi- 

 tive fishes, the sharks, we find that the skeleton consists of cartilage 

 alone. It is divided into axial, appendicular, and visceral parts 

 as in the rest of the vertebrates but the structure of each part is 



