510 MANUAL OF ELEMENTARY ZOOLOGY 



has no blastoccele, but contains yolk. A shallow invagina- 

 tion on one side of this gives rise to a gastrula with a small 

 enteron. The mesoderm arises as two ventral bands, 

 though pole cells are not found. The later development 

 of these animals cannot be followed here, but it may be 

 stated that it is quite unlike that of the Vertebrata. 



A comparison of the processes that have just been de- 

 scribed shows two facts of importance, (i) They 



Von Baer's Law. ... . c * \ \ m. 



all have certain features in common. (2) I he 

 animals which are more alike as adults resemble one 

 another longer in development. This generalisation is known 

 as von Baer's law. All animals have at one stage a single 

 nucleus. All Metazoa pass later through a gastrula stage 

 of two layers only. When the Triploblastica acquire a 

 third layer, Annelida, Arthropoda, and Mollusca have in 

 common a process in which it starts as ventral bands, 

 while in all Chordata it arises from the dorsal side of 

 the primitive gut. All Chordata also have at one stage 

 a notochord, a hollow dorsal nervous system formed 

 by the folding of a neural plate, and gill-clefts. All 

 Vertebrata have at a later stage a cartilaginous skeleton 

 and a circulatory system like that of a fish. At a later 

 stage still, the frog, bird, and mammal have pentadactyle 

 limbs and the rudiments of lungs. The embryo of a 

 rabbit is at one stage much like that of many other 

 mammals, then it takes on the features of a rodent, 

 finally it shows those of its own species. At the same 

 time it must not be overlooked that von Baer's law holds 

 good only in a very general sense. The resemblance 

 between the young stages of related animals is never exact 

 and is often greatly obscured by disturbing factors, such as 

 variations in the amount of yolk present or the precocious 

 development of certain organs. Thus, for instance, the 

 two-layered stages of the lancelet, frog, and chick (Figs. 

 359 '> 3 6 9 '> 384, 2) are extremely unlike on account of 

 differences in the amount of yolk they contain, and again 

 amnion and allantois, which are peculiar to reptiles, birds, 

 and mammals, are developed at an exceedingly early stage, 

 when the embryo is only beginning to take on the features 

 which are common to all chordate animals. 



