[75] EMBRYOGRAPHY OF OSSEOUS FISHES. 529 



the mjestion of food ; this secretion iu such eml)ryos is probably analo- 

 gous to the meconium discharged by recently born infants. 



Meanwhile the lumen of the oesophagus, pharynx, and mouth are 

 being differentiated. In sections at this stage the oesophagus has a 

 lumen, and is not solid, as Balfour states (Comp. Embryo!., II, 63), but 

 is depressed or cylindrical at its hinder part, while beneath the head it 

 rapidly widens, where its width exceeds its depth several times. In the 

 cod, however, its anterior flattened portion is short, and is not so ex- 

 tended as the same part in embryo Clupeoids. This flattened anterior por- 

 tion of the mesenteron is molded upon the lower face of the brain, and 

 is concave from side to side on its upper surface and convex from side 

 to side on its lower. Its walls are very thin in contrast with the more 

 posterior portion of the intestine or mesenteron, and are hardly more 

 than one layer of cells deep in places. In longitudinal sections of em- 

 bryos of the Clupeoid Alosa, in which the mouth is just about to break 

 through, the most anterior or hyomandibular cleft which intervenes 

 between the hyoid and mandibular arches seems to be the most devel- 

 oped, but it does not appear to break through the skin. Behind this the 

 six gill-clefts are developed on either side of the pharyngeal portion 

 of the fore- gut. They appear to be of the nature of narrow lateral paired 

 outgrowths from the sides of the depressed fore-gut, and have at first 

 only a very narrow cleft-like lumen. The gill-clefts 4ire at first very 

 much crowded together antero-posteriorly in the young just-hatched 

 cod, as may be gathered from Figs. 40, seen from the side, and 46, viewed 

 from below, where the gill-clefts are shown at g. Dohrn holds that the 

 mouth is to be regarded as an anterior outgrowth of the mesenteron 

 from behind forwards, that it is divided in the middle line, and that the 

 two limbs of the larval mouth grow out laterally and separately. He 

 also seems to regard these paired oral outgrowths as the first of the 

 branchial clefts, counting the second as the hyomandibular. As already 

 stated, I have not been able to fully convince myself that this is the 

 fact, although I have seen evidence in a series of sections of embryo 

 Olupeoids which have inclined me to think Dohrn's view the correct one. 



The mouth breaks through iu or near the angle formed by the lower 

 fore part of the head and the anterior epiblastic wall of the yelk-sacl^ 

 at the point m in Fig. 40. In Alosa the point where the superficial ex- 

 ternal epiblast is continued into the oral hypoblast is exactly in the angle 

 alluded to above, and, as far as I can make out from longitudinal sec- 

 tions, there is no clear evidence of a distinct epiblastic oral invagina- 

 tion or stomoda3um, such as is found in Petromyzon, for example. As de- 

 velopment advances, the upper lip grows forward in advance of the 

 end of the lower jaw to a marked extent, exposing the roof of the larval 

 mouth considerably. The lower jaw, after this, begins to elongate, and 

 soon grows in length so as to regain what it had apparently lost in 

 relative length as compared with the u])per. It is during theearly stages, 

 before the outgrowth of the lower jaw, that the mouth gapes, the man- 

 S. Mis. 46 34 



