THE CEPHALIC NAVEL. 25 



arachnids, an extra embryonic germinal area, and that certain parts of this area 

 may be regarded as the beginning of an extra embryonic vascular area. The 

 peripheral ends of the vagus and abdominal lateral plates give rise to the heart, 

 pericardium, longitudinal haemal muscles, and to blood corpuscles. 



Concrescence. As the lateral margins of the germinal area grow faster 

 than the median portion, concrescence of the germinal wall will ultimately occur 

 in the precephalic and post caudal regions. In very large yolked eggs, pre- 

 cephalic concrescence will tend to bring the cardiomeres into conjunction, either 

 in front of, or underneath the procephalon, that is in their characteristic position 

 invertebrates. (Figs. 17-23, 138, 140, 141.) 



The post caudal concrescence will tend to unite the posterior margins of 

 the germ wall behind the real apex of the body, giving rise to the various 

 phenomena in vertebrates that have been confused with "gastrulation," "con- 

 crescence of the lips of the blastopore," and with apical growth. 



The New Mouth, Cephalic Navel, or Haemastoma. 



In the arachnids, there is a special area on the anterior haemal surface, 

 just in front of the procephalon, that we shall call the cephalic navel. It prob- 

 ably occurs in all arthropods, under various modifications, as the so-called dorsal 

 organ. It is primarily a thickening of the haemal blastoderm, entirely outside, or 

 beyond the germinal area. In the arach- 

 nids, the thickened blastoderm gives rise 

 to an immense mass of proliferating cells 

 that are ultimately invaginated into the 

 yolk, where they degenerate and are absor- 

 bed. This infolded area of degenerating 

 cells forms the central point toward which 

 all the surrounding organs converge; the 

 germ wall, with its appropriate structures 



advancing toward its sides and posterior \ *** 1! J^-^^^^^^ i 

 margin, and the procephalon toward the <S2sr~^ I^T^^ 



anterior one. There is thus formed, either 



in front Of, Or below, the procephalon, a FIG. i8. Anterior end of an embryo scorpion, 



j -i i n ,v showing forebrain completely covered by the palial 



vortex center toward which all the sur- fold 



rounding organs move, and into which is 



infolded the remnants of the haemal blastoderm (Figs. 23, 127, 138, 139, c.nv.) 



The opening between the enteron and the exterior, thus virtually established on 



the haemal surface, finally closes in the arthropods, but in the vertebrates a 



permanent opening is established at this point, that becomes the new mouth, or 



the hcemastoma. 



Although the cephalic navel ultimately closes in the arthropods, prophetic 

 signs of its future function are not lacking, for on the site where it is formed, 



