The Dcvelopiiicnt of the Heart in Shad, 



249 



Stage of 7.3 mm. 



Fig. 15 is a diagram, made in the same way as Fig. 14, from a 

 reconstruction of the pericardial region of an embryo of 7.3 mm. 



The head fold proper no longer exists, having been absorbed into 

 the basal ectoderm of the surface. The pharynx is dilated, and 

 the oral plate is soon to be perforated. Antero-posterior compres- 

 sion of the head is still more marked than in the preceding stage, 

 and is accompanied by the formation of a head-bend of the mid-brain 



-'^NSiic of entrance 

 (o atrium 



Fio. 15. — Diagram made from a reconstruction of tlio pericarilial region 

 of shad, stage of 7.3 mm. x 100 diams. 



Tlie left side is shown, structures indicated as in the preceding figure. 



and of a curve, in the ventral direction, of the anterior end of the 

 notochord. There is an ectodermal band connecting the lateral 

 margin of the anterior end of the entodermal pharynx, on either side, 

 with the basal ectoderm of the surface. The ectodermal bands re- 

 ferred to represent the posterior margin of the original head-fold 

 ectoderm, and have a developmental history similar to that of the 

 head-fold proper (see footnote on page 237.) These ectodermal 

 bands and the mandibular anlage (combined entodermal and ecto- 

 dermal mandibular pouches) are now undergoing disintegration. 



The heart differs from that of the preceding stage in that the 

 apex of the ventricle is well-marked. 



Fig. 15 shows (compare with Fig. 14) that the ventral layer of 

 the head-fold, in completely separating from the dorsal layer, has 



