62 DEVELOPMENT OE THE STAEFISH SOLASTEE ENDECA. 



PLATE III. Figs. 24-35. 



Fig. 24. Sketch from cleared specimen at stage in middle gastrulation with one of the 

 large surface-furrows still present and leading into the blastopore, which is 

 widely open. The invaginated hypoblast is seen in darker shading as a 

 slightly folded layer within the epiblastic layer, (x 35.) 



Fig. 25. Optical section of stage immediately succeeding gastrulation. The blastopore 

 is narrow; the archenteron has expanded and shows in its lining the 

 thickened zone A B, from which, as was explained on p. 37, the right and 

 left lateral diverticula of the anterior ccelom, as well as the walls of the 

 enteron, will be derived. (x30.) 



Figs. 26-27. Two optical sections of a stage slightly later than the last, fig. 26 being a 

 view from the left, and fig. 27 a view from the dorsal aspect and slightly 

 from the right. Both show elongation of the gastrula-cone as well as 

 commencing formation of the ventral dimple. Fig. 26 shows also the slight 

 obliquity of the base of the cone referred to on p. 37. It will be observed 

 that the posterior part of the cone is now somewhat narrower in the frontal 

 than in the sagittal plane. The constriction marking the enteric folds is 

 not apparent in the lateral view (fig. 26), since it is actually present as yet 

 only at the sides. ( X 30.) 



Figs. 28-30. Three sections from a frontal series of a young larva about 24 hours older 

 than that shown in the last two figures, but with the blastopore still open 

 and the enteron still in the form of an isthmus between the anterior and 

 the posterior coeloms. The sections illustrate particularly the difierent 

 sizes of the right and left enteric folds dorsally, mesially, and ventrally, 

 throughout the series. It will be observed that dorsally (fig. 28) the right 

 fold (r.g.), though less prominent, is larger than the left one ; ventrally 

 (fig. 30) the condition is reversed, though with smaller diffisrences, while 

 mesially (fig. 29) the right fold is still slightly larger, (x 50.) 



Fig. 31. Frontal section of older larva. The apertures between the middle and 

 posterior chambers are just closing at the points which the section cuts. 

 It will be seen that the right gastric fold is contributing rather more than 

 the left one to the total area of the enteron, and also that there is a slight 

 pouching to the right of the enteron-wall. ( X 45.) 



Fig. 32. Similar section to the last, but of a slightly older larva. The middle chamber 

 is now quite closed off" from those in front and behind. In front, however 

 the seam of the closure is still recognisable looking leftward. The same 

 slight pouching to the right on the part of the enteron is seen here as in 

 the fig. 31. (X45,) 



