DEVELOPMENT OF THE HEAD IN GOBIUS OAPITO. 343 



the left, occurs a considerable ingrowth of ectoderm, 

 which represents the maximum ingrowth of the mouth at this 

 stage. At this spot it cannot be perceived exactly how far the 

 ectodermic ingrowth extends. In fig. 10, which represents a 

 section 25 fi further posteriorly (5 sections), the limits can be 

 sharply defined, and the ectodermic ingrowth is seen to be 

 connected only with the ectoderm of the skin by a neck, while 

 towards the middle line it touches another solid mass of cell, 

 representing a solid forward growth of endoderm. 



In fig. 9 ectoderm and endoderm are fused. Where the cells 

 border on the yolk they form a more or less definite layer. In 

 all the sections anterior to that represented in fig. 10 this layer 

 can be perceived, and it separates brain and mesoderm from the 

 yolk. It is very distinct on the right hand in fig. 9. It can be 

 seen to pass insensibly into the external ectoderm, which forms 

 the lens of the eye. In the same figure the outer layer or 

 '' Deckschicht " of the ectoderm has been separated from the 

 lower layer. Under the centre of the eye no marked solid 

 ingrowth of the ectoderm occurs. 



Fig. 8 is a drawing of a section further forward through the 

 posterior limit of the optic stalk. Its position is indicated ou 

 the diagram by the line c. Here, again, is seen solid ectoderm 

 {Hyp.}, occupying a portion of the angle between the eye and 

 brain. Further forward (fig. 7, on the left side) the ectoderm 

 is indistinguishable from the mesoderm surrounding the eye. 

 This mesoderm extends forward below the eye-stalks. At the 

 anterior end of the embryo the thickening of ectoderm {Hyp.) 

 is continuous with the ectoderm of the nose. 



From these observations one may learn that the embryo at 

 its anterior end is separated from the periblast and yolk by 

 continuous ectoderm, one cell thick in the median region, and 

 that two paired thickened ingrowths occur, the anterior pro- 

 ceeding inwards from the region of the nose to below the optic 

 stalk, and the posterior proceeding directly towards the middle 

 line below and behind the eye. The anterior ingrowth, which 

 is to a certain extent continuous with the ectoderm of the nose, 

 gives rise, as we shall see later, chiefly to the hypophysis ; while 



