132 EMBRYOLOGY 



marcated from the body, and the eye, nose, suckers, gill plate, pronephric 

 swelling, and somites are discernible by external inspection. Figure 73 shows 

 three stages of development — almost stage 18, stage 18, and a very late stage 

 18. Notice the increase in length of the tail bud, the increasing differentiation 

 of the head region, and the changes in the form of the gill plate. The ecto- 

 derm is covered with cilia (not shown) at this time, and the embryo moves 

 inside the jelly. At about stage 18 the embryo is able to contract its muscles 

 when stimulated. During these stages of development the embryo is still 

 within the jelly and membrane, which swell and take up water from the 

 surroundings. The embryo thus develops in a small droplet of water encased 

 in a protective membrane. 



These changes may be seen by comparing two stages in the development 

 of the salamander embryo (Fig. 74). In the older embryo note the increase 

 in over-all length, the better developed head. The posterior markings, which 

 are caused by the somites, and the increased length of the tail are also shown. 



Structure of the embryo at hatching 



To continue with our survey of the development of the frog embryo we 

 shall consider stage 20, which is shown in Figure 75. The tail has increased 

 in length and the head is more prominent. The gills begin to grow out from 

 the gill plate and show branching. Blood corpuscles circulate through the 

 gills at this time. The heart began to beat at stage 19. The head view shows 

 the prominent suckers, which secrete a mucus used to attach the embryo to 

 waterplants after hatching. At about this time the embryo emerges from the 

 jelly, which has been previously softened and dissolved by a hatching enzyme. 

 The mouth has begun to form by an invagination, the stomodeum. Reflex 

 movements continue to develop, and at hatching the embryo is able to swim. 



Let us now study the internal structure of the frog embryo at the time 

 of hatching, stage 20. We shall examine the structures found in cross sections 

 of the embryo and shall begin with the most anterior sections and work 

 posteriorly. In discussing each structure we shall first outline its origin, then 

 describe the condition of the structure at stage 20, and finally discuss its 

 subsequent fate and function. 



In the first section (Fig. 76) the large, thick-walled forebrain (prosen- 

 cephalon) is most conspicuous. The forebrain arises from the most anterior 

 region of the neural plate and gives rise in subsequent development to (l) 



