246 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



mistaken for an extension of, or as a product of, apical growth. This is not the 

 case. The yolk cells, or endoderm cells, formed around the posterior end of the 

 primitive streak and from the walls of the telopore, belong to the primitive germinal 

 area, and are the products of true apical growth. The post-anal proliferation in 

 the caudal navel is of a different nature. It represents an isolated part of the 

 degenerating haemal blastoderm, enclosed by a narrowing area formed by the germ 

 wall. The area is finally closed over by the definitive ectoderm, leaving behind 

 a special cloud of mesoderm and yolk cells formed by the united germ walls. 



The conditions just described for Limulus and other arachnids give us the clue 

 to the correct interpretation of the phenomena of concrescence in the vertebrates. 

 Here there is no doubt a true axial, or apical growth, and a false axial growth 

 formed by the post-apical concrescence of two bands representing the proliferating 

 margins of the germinal area. But in vertebrates it is difficult to distinguish be- 

 tween that part of the embryo formed by apical growth and that formed by con- 

 crescence, and it has been assumed that there is no real distinction between them. 

 The primitive streak of vertebrates for example is often regarded as an ancient 

 line of concrescence, and the real concrescence that takes place behind it, as a 

 continuation of the same process, more formally expressed. Both the primitive 

 streak and the actual line of concrescence are supposed to represent different phases 

 of "a modified method of uniting the lips of a greatly elongated gastrula mouth." 

 Minot. Embryology, p. 126. But according to our interpretation, there is no 

 remnant whatever of a gastrula mouth at the caudal end of any segmented animal. 

 The real apex of the body is an actively growing point composed of proliferating 

 teloblasts that give rise to the axial parts of the body. True axial growth cannot 

 take place by concrescence, because the parts thus united represent the extreme 

 lateral or haemal ends of the metameres forced into the neural, or axial, position 

 by the peculiar exigencies of apical growth on a spherical surface. 



We may recognize in vertebrates, as in arachnids, an axial telopore, or primi- 

 tive streak, and post-apical concrescence of the margins of the germinal area. 

 But these modes of growth are so blended with one another in vertebrates that it 

 is extremely difficult to tell where one begins and the other ends. 



The actual separation of the products of teloblastic growth varies widely in 

 different segmented animals. But the differences are in degree, or in method, not 

 in kind, or in end results. For example, in forms like Cymothoa, there is a trans- 

 verse row of large superficial teloblasts, which like the cells at the apex of a growing 

 plant stem, give rise in the most precise and regular manner, to the various parts 

 of the trunk. 



In Limulus, in birds, reptiles, and mammals, there is a true axial infolding or 

 primitive streak, and the various products of apical growth that extend forward 

 from it may be recognized almost as fast as they are laid down. In amphioxus 

 and the tunicates, there is a much more extensive apical infolding, and the products 



