ORIGIN OF THE HEART 



573 



stage). The distance between the rudiment of the amn ion-fold and 

 the lateral edge of the primitive band (*x, *y) is occupied by an epi- 

 thelial lamella (I), in which we recognize the earlier amnion. This 

 lamella does not lie directly on the yolk, but is separated from it 

 by a spacious blood-lacuna (A, bs), in which can be seen numerous 

 blood-corpuscles which have migrated in from the mesoderm of the 

 primitive band. The cardioblasts which have arisen from the wall 

 of the primitive segment (MS) are on each side arranged into the 

 form of a furrow (gr), which bounds the blood sinus below. 



f 



. 



I'M 



Fio. 549. Cross-section through the abdominal part of an older primitive band of P. ger- 

 mnnicn when beginning to grow around the yolk: rm, ventral longitudinal muscle; other letter- 

 ing as in Fig. 545. After Heymons, from Kor'schelt and Heider. 



By the continuous growth of the primitive band around the yolk, 

 after the resulting invagination and degeneration of the dorsal plate, 

 the two blood-lacunse unite together on the dorsal side into a single 

 one (B, 6s). These constitute the first cavity of the heart. The 

 vascular furrows (gr~) come in contact with each other and grow 

 together, and the wall of the heart is thus formed. Ayers states 

 that in CEcanthus the heart is formed in the head region only after 

 the yolk-sac has passed entirely within the body. The venous ostia 



