224 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



nids presents an interesting intermediate series of stages between the typical cen- 

 trolicithal type of insects and that of the lower vertebrates. 



In Limulus, there is a distinct approach toward the partial cleavage of the 

 amphibia and cyclostomes. In Telyphonus (Schimkewitsch), the form of the 

 blastomeres, and the size and location of the resulting segmentation cavity are 

 very similar to the corresponding structures in the frog's eggs. 



In the scorpion, which possesses one of the largest eggs among the arthropods, 

 alt the early divisions take place on the surface of the yolk (Brauer), producing 

 a typical meroblastic cleavage, and a small sharply defined blastodisc very similar 

 to that in the teleosts. 



In many insects and spiders, the early divisions take place in the interior of 



FIG. 123. A, Surface view of the blastoderm and primitive cumulus of Limulus, showing the beginning of 

 gastrulation, a.c., and the formation of the primitive germinal area, p. b. I; B, later stage showing the increasing 

 number of inner layer cells that mark the boundaries of the germinal area, a.nd the formation of the posterior 

 cumulus, p.c., that marks the beginning of the trunk, and of teloblastic, or apical, growth. 



the egg, and all the nuclei thus produced may move to the surface to form the 

 blastoderm, from which yolk cells and mesentoderm cells arise by a subsequent 

 process of division and ingrowth. But in some insects and arachnids, cleavage 

 nuclei remain in the interior of the egg as the so-called yolk nuclei, which, as a 

 rule do not give rise to the definitive endoderm. In Limulus, the condition appears 

 to be exceptional, in that most of the yolk cells derived from the early cleavage 

 nuclei, persist as the permanent lining of the midgut and its diverticula. 

 Although accessions are subsequently made to the yolk nuclei by the ingrowth 

 of cells from the germinal disc and germ wall, no definite bands of columnar 

 endoderm cells, such as those seen in insects and Crustacea, are formed. 



On the whole, the cleavage seen in the arachnids (Limulus and the scorpion) 

 is very similar to that of primitive vertebrates, and affords us a satisfactory basis 

 for the interpretation of the later stages of development in the arthropod and 

 vertebrate stock. 



2. The Germ Disc or Primitive Cumulus. 



The embryo first appears about four and a half days after fertilization, as 

 a very faint, white, germinal spot. Later, it may be a minute, roughened papilla, 



