228 H, D. Senior. 



in process of an entire alteration in its distribution; it now consists, 

 on either side, of three parts: One part still remains dorsal to the 

 lateral plate, a second is on tlie ventral surface of this structure, 

 and a third forms an isthmus between the other two and occupies 

 the descent area. From now on it will be convenient to speak of 

 the part ventral to the lateral plate as endocardium (for such it 

 really is) and to retain the original term for the part which is still 

 dorsal to the lateral plate and for the isthmus. The endocardium 

 now appears as two patches (colored blue in Fig. 4) ventral to the 

 entodermal pharynx and the lateral plates. These patches are not 

 limited to the descent area, but are spreading backwards (more so 

 on the left side of the embryo than on the right, see Fig. 4B and 40). 

 The backward (caudad) movement of the endocardium, which is 

 now beginning, is soon to become very pronounced. The asymmetry 

 seen in this reconstruction is thought to be due to unequal growth 

 of the embryo, rather than to faulty building up of the plates, the 

 entire right side of the head appears to be in a less advanced stage 

 of development than is the left. 



In an embryo of 22 somites (Fig. 5, one hour and a half older 

 than the preceding stage) there has been considerable advance in 

 development. Ventral closure of the pharynx has progressed rapidly 

 from before backwards, and is also beginning in another place pos- 

 terior to the gill-region. The medial margins of the lateral plates 

 are approaching one another, and the notochord, slightly longer than 

 before, is now fully formed as far as its anterior end is concerned. 

 The endocardium has travelled back to a point posterior to the 

 mandibular pouch (Fig. 5D), and the portions arising from each 

 side have met across the mid-line. Fig. 5B (as compared with 4B) 

 shows that the descent area has extended considerably backwards, 

 but that descent is not yet occurring opposite the apex of the 

 mandibular pouch is shown on the left side of Fig. 50 (right side 

 of embryo). 



At a stage of 26 somites (Fig. 6, one hour and a half later than 

 the preceding stage) the ventral closure of the pharynx is approach- 

 ing completion ; rapidly, however, as closure of the pharynx is taking 

 place, it has been overtaken by the backward growth of the endo- 



