236 



EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



Assheton 1 has described some peculiar wandering cells, of unknown fate and 

 significance, that appear in many parts of the body on the sixth and seventh days 

 of incubation in Gymnarchus nitolicus, p. 369-370. In their distribution, size, 

 "peculiar refrangent qualities" and in the "ribs which run from pole to pole," 



FIG. 138. Schematic diagrams in mercator projection, at two different stages illustrating the arrangement and 

 mode of growth of the mesoderm and its derivatives in an arachnid embry o . 



they greatly resemble the fiber cells of the arachnids and I have no doubt they 

 are in reality the same kind of cells. 



Similar cells have also been described by Plehn, 1906, in teleosts. They are 

 oval, thick-walled cells with a small excentric nucleus and with numerous fine, 

 highly refractive, unstainable rods or threads, all converging toward the non- 



A. 



B. C 



FIG. 139. Schematic figures of the haemal surface of arthropod and vertebrate embryos, to illustrate the 

 method ol closing the haemal surface in large yolked embryos. A, Insect type, with elongated cephalic navel; 

 B, arachnid type with larger yolk sphere and overhanging or projecting cephalic and caudal lobes; C, vertebrate 

 type, with a relatively larger, more posteriorly located, yolk sphere; with cephalic appendages concrescing around 

 the cephalic navel; and with delayed concrescence of the centrally located cardiomeres, thus giving rise to the 

 bifurcated heart tube, and to the middle, or belly, navel. 



nucleated pole of the cell. They are widely distributed in the walls of blood ves- 

 sels and in lymphoid tissue, and are said to form a secretion that is probably 

 emptied into the blood. See also Phoronis and Lernaea, p. 447. 



Vascular Area. From the preceding descriptions it will be seen that an ex- 

 tra-embryonic area is established in the arachnids in which we may recognize the 



1 The Development of Gymnarchus Niloticus. The work of John Samuel Budget, Cambridge, 1907. 



