BULLFROG AS TYPICAL VERTEBRATE ANIMAL 101 



cells. This sheet of cells is known as mesoderm. As the mesoderm 

 grows it splits into two sheets, an outer, or somatic layer, which lies 

 next to the ectoderm of the embryo wall, and an inner, or splanchnic 

 layer, which lies next to the endoderm cells of the archenteron. The 

 cavity formed between these two layers is the beginning of the coelom, 

 or body cavity. From these three layers, ectoderm, endoderm, and 

 mesoderm, all the body structures are formed. 



Formation of Nervous System. — With the reduction of the blasto- 

 pore to a very small area, there appears on the dorsal side of the 

 embryo a thickened plate of ectoderm known as the neural plate. 

 This plate soon is flanked on each side and in front by ridges known 

 as neural folds. As these folds arise, the remainder of the plate 

 forms a groove known as the neural groove. The neural folds or 

 lateral edges of the plate curl in and meet in the dorsal midline, where 

 they fuse to form the neural tube. The neural plate is wider at its 

 anterior end than it is nearer the blastopore, and at this expanded 

 anterior end the brain arises, while the remainder becomes the spinal 

 cord. 



During these changes, the egg, which previously was spherical, has 

 elongated in the axis of the neural tube. The blastopore has been 

 covered by the folding of edges of the neural groove extending to 

 its borders. Subsequently the embryo takes on form so that defi- 

 nite body regions can be identified. 



Later Development. — In the head region appears an elevated side 

 plate, the gill plate, where later the gills develop. Anterior to this, 

 a swelling on each side of the head denotes the beginning of certain 

 sense organs. A depression on the anterior ventral surface is a fore- 

 runner of the mouth, and posterior to this a crescent-shaped area 

 indicates the beginning of a ventral sucker by which the newly 

 hatched larva may attach itself to objects. At the posterior end of 

 the body a tail hud appears, and the region of the blastopore becomes 

 the anus. The embryo soon hatches, branched external gills which 

 serve as respiratory organs make their appearance, and definite sense 

 organs can be found on the head. This is the external gill stage. 

 The intestine becomes coiled ; the tail elongates ; and muscle segments, 

 or myotomes, can be seen along the sides of the body. Shortly after 

 hatching, the external gills are absorbed and internal gills take their 

 place. A fold of skin, the operculum, develops over this region. 

 There remains on the left side, however, a small opening, the spiracle, 

 through which water may pass out after coming into the mouth over 



