240 EARLY STAGES OF ARTHROPOD AND VERTEBRATE EMBRYOS. 



finally disappears. The coarse irregular nuclear masses break up into very fine 

 granules, which become scattered through the yolk and absorbed. 



Meantime, as the germ wall passes the equator of the egg and advances to- 

 ward the anterior haemal surface, it surrounds a gradually narrowing area, where 

 the degenerating blastoderm cells are being crowded into the yolk and overgrown 

 by the germ wall and its products. The ingrowing blastoderm, and the narrowing 

 ring formed by the germ wall and vascular area, constitute the cephalic navel. 

 (Figs. 139-149, c.nv.) 



When the vascular area and germ walls finally close in the haemal surface, 

 the entire extra-embryonic blastoderm and its products have disappeared in the 

 interior of the egg. Owing to the shape of the yolk sphere, and to the unequal 

 expansion of the thoracic and abdominal lateral plates, the greater part of the 

 blastoderm is crowded into the anterior portion of the mesocephalon, and is last 

 seen disappearing into the yolk just behind the procephalon. (Figs. 149, 150, 

 c.nv.) 



In stage O, the last remnants of these cells may be seen scattered about in the 

 yolk contained in the first four pairs of enteric pouches. (Fig. 151.) 



Other Arthropods. — The cephalic navel of Limulus without doubt represents 

 one phase of the structure familiar in insects, Crustacea, and myriapods, and which 

 is usually spoken of as the "dorsal organ." It has only recently been recognized 

 in the arachnids. I have found a similar structure to that of Limulus in the 

 scorpion, and, according to Schimkewitsch, one is found in Pholcus and 

 probably in Telephonus. One has also been described in the copepods. 

 (Fig. 272, B.) 



In the Crustacea, the conditions centered in or around the cephalic navel 

 give rise to a variety of structures. It may be a transitory, embryonic gland- 

 like organ, as in isopods; a larval organ serving for temporary attachment, as in 

 cladocera (Figs. 8, 9) ; or a voluminous outgrowth that serves throughout life for 

 the attachment of the animal to some inanimate object, or to its host, as in cirri- 

 peds and parasitic copepods. (Figs. 275, 282, 283.) 



The cephalic navel, in one form or another, is therefore found in all classes 

 of arthropods. What its original significance may be is not apparent. But its 

 function, when it has one, and its location and general mode of growth are 

 constant. 



There are clearly two factors involved in its formation: i. the low pressure 

 area formed by the degenerating haemal blastoderm; and 2. the convergence of the 

 margins of the germinal area toward the center of degeneration. The invagination 

 of the degenerating blastoderm into the yolk, and the closure of the germinal 

 area around it, gave rise to a fistula-like communication between the enteron and 

 the exterior. This opening may close up completely during the embryonic stages, 

 leaving no scar behind (insects and arachnids); or around the point of closure 

 scar-like glandular structures or outgrowths may develop that serve as temporary 

 or permanent means of attachment (crustacea, cirripeds), or in parasitic forms 



