"742 Bashford Dean Memorial Volume 



anlage of the epitheKal hypophysis. It extends from the oral ectoderm to a slight de- 

 pression in the floor of the diencephalon — an evagination which may be the rudiment of 

 the infundibulum. 



Figure 24, plate II. represents in surface view an embryo with at least 28 somites. 

 Unlike the preceding embr>^os, this one wd.s drav^Ti from the left side and has been re- 

 produced with right and left sides reversed to facilitate comparisons with other figures 

 on the same plate. It is evidently drauTi to the same scale as Figures 21 and 22 (same 

 plate). There is some increase in length but Httle advance in external differentiation. 

 However, spiracular and first branchial grooves begin to look Hke gill-clefts. This is the 

 first drav,ing to show distinctly the region of the heart — ^just in front of the broad yolk 

 stalk. There is a sHght caudal flex-ure. The angular projection of the forebrain is probably 

 abnormal. 



Figure 25, plate II, a surface view, portrays an embryo with about 35 complete 

 somites. Like the preceding figure, this one w^s drav.Ti from the left side and has been 

 reproduced v.-ith right and left reversed. Here, for the first time in this series, we find 

 the sites of the spiracular cleft and the first and second gill-clefts sharply defined. It is 

 not certain that the closing plates between branchial grooves and pharyngeal pouches are 

 already perforated, but this seems a good place to begin referring to these fissures as 

 clefts instead of grooves. The cer\ncal flexure is pretty well straightened out, but in 

 the cephalic flexure there is not much change. The dark spot dorsal to the first gill-cleft 

 (not the spiracular cleft) indicates the site of the otic vesicle. The region of the heart is 

 clearly indicated. 



Figure 26, plate II, pictures a cleared embr>'o unth about 37 complete somites and 

 one or two incomplete somites at the anterior end of the series. This drauing appears to 

 have been made at a higher magnification than the preceding figure. The principal divis- 

 ions of the brain are fairly well shouTi, though not so clearly as in Figure 20 of the same 

 plate. One notes, in the branchial region, the entoderm-lined spiracular cleft and 

 gill-clefts alternatmg with mesodermal visceral arches. The greater size of the spiracular 

 cleft is noteworthy, considering its small size in the adult. In its early stages it looks like 

 a gill-cleft, and indeed it is homologous with the gill-clefts. The notochord is clearly 

 visible throughout most of its length: it ends anteriorly just above the spiracular cleft. 

 Almost at the tip ot the tail bud, the neurenteric canal is sharply outHned. Though the 

 hind-gut is rather vaguely defined, there is a distinct posterior intestinal portal. Auricular 

 and ventricular divisions of the heart can be distinguished. The optic cup show^s a very 

 thin outer and a thick inner layer; these layers are united along the borders of the chorioid 

 fissure. The otic vesicle, dorsal to the first gfU-sHt, is quite prominent. 



In front of the dorsal part of the spiracular cleft and close to the brain (Tigure 26, 

 plate IT) there is a thick-Vv-alled roughly circular sac which may be a "head cavity" or 

 head somite. In Elasmobranchs and perhaps in vertebrates generally, the muscles that 

 move the eyeball arise (^larshall, 1881: Van Wijhe, 1882; Scammon, 1911: Neal, 1918) 

 from mesodermal segments (head somites i which are serially homologous with the somites 



