EARLY DEVELOPMENT OF THE FROG. 59 



cells of the entoderm is the food that must suffice until the animal reaches a 

 stage when it is able to obtain a supply from the outside; but the rudiment 

 of the future complex alimentary mechanism is already formed. The 

 blastopore is not a free opening, as in Amphioxus, but is obstructed by the 

 yolk plug. The latter is eventually withdrawn and the anus develops in 

 the site of a part of the blastopore. The mouth is a new opening which 

 develops at the forward end of the gut. A somewhat more detailed discus- 

 sion of the biological significance of the blastula is given on page 40, in the 

 chapter on Amphioxus. 



Mesoderm Formation. In order to detect the beginning of the middle 

 germ layer it is necessary to look back into the period of gastrulation. Gas- 

 trulation and mesoderm formation overlap each other. In a sagittal section 

 of the blastula just as gastrulation commences, the cells of the germ ring 

 are continuous with the yolk cells above the groove that indicates the begin- 

 ning of in vagina tion (Fig. 31, A). This transition zone, traced through the 

 subsequent stages of development, is composed of cells which occupy a posi- 

 tion always in the angle between ectoderm and entoderm and merge with 

 these layers (Fig. 31, B, C, D, E, F). The cells in question comprise the 

 early mesoderm. Appearing as it does in the angle between the other layers 

 in the lip of the blastopore, it is obvious that when the blastopore becomes 

 circular the mesoderm takes the form of a circular band. In Amphioxus it 

 was clear that the mesoderm originated from entoderm (see p. 42), but in 

 the frog the first mesodermal cells bear such relation to the other layers that 

 their origin is not so readily determined. In later stages, however, it will 

 be apparent that mesoderm arises from yolk entoderm. 



In the description of gastrulation it was pointed out (p. 58) that during 

 the migration of the crescentic groove and its transformation into a ring 

 the cells along the horns of the crescent were drawn medially and piled up in 

 an axial strand which then extended upward and forward from the dorsal lip 

 of the blastopore. The mesodermal cells appear in the dorsal lip of the 

 crescentic groove and, as the migration of the groove goes on, they are 

 affected in the same way as the other cells in this region. Therefore the 

 band of mesodermal cells around the blastopore is broader at the dorsal side. 

 In other words, a band of mesodermal cells extends upward and forward 

 from the dorsal lip of the blastopore, forming a part of the axial strand. 

 And since the proliferation and involution of cells, which occur during gas- 

 trulation, tend to carry the mesodermal cells upward and forward and since 

 the mesodermal cells themselves are proliferating, the mesoderm soon 

 becomes almost as extensive dorsally as the entoderm. 



In the dorsal axial strand of cells, which later will be considered more in 

 detail, the three layers are at first merged. Lateral to this the mesoderm 



