xm PRACTICAL DIRECTIONS 615 



of the allantois is represented by a cord or ligament, the 

 urachus, which connects the navel with the apex of the 

 bladder, so that only a small portion of the allantoic 

 outgrowth and not the whole of it, as in the frog, 

 persists in the adult. 



PRACTICAL DIRECTIONS 



A. A series of models of the development of Amphioxus, 

 to be found in most Zoological Museums, should be 

 examined. Note 



a, the various stages of segmentation and the formation 

 of the segmentation-cavity (Fig. 152, A-G) ; b, the oblitera- 

 tion of the segmentation-cavity during the formation of the 

 gastrula, with its ectoderm, endoderm, and blastopore (H, I) ; 

 c , the elongation of the gastrula, the flattening of the surface 

 which will become dorsal, and the position of the blastopore 

 at the posterior end (J) ; d, the formation of the medullary 

 plate and cord, the mesodermal segments and enterocceles, 

 and the separation of the latter from the enteron, the walls 

 of these cavities forming the parietal and visceral layers of 

 the mesoderm ; and the formation of the notochord and the 

 neurenteric canal (K O). 



B. In order to follow out the development of the chief 

 organs in the Vertebrata, it is necessary to make a number 

 of serial sections of various embryonic stages. For this 

 purpose chick-embryos (see below) are, on the whole, the 

 most convenient and satisfactory ; but if you have not 

 already followed out the instructions given on pp. 212-214, 

 you should not only examine externally the stages in the 

 development of the tadpole up to hatching, but also attempt 

 to make sections of some of the early stages so as to show 

 a, the segmentation-cavity (Fig. 64, E) ; b, the enteron, 

 blastopore, and yolk-plug (I) ; and c, the medullary cord, 

 notochord, mesoderm, and ccelome (Figs. 64 K, and 65, A 

 and B). Compare with the corresponding stages in 

 Amphioxus. 



C. A number of fresh, impregnated fowls' eggs should be 

 obtained and placed in an incubator at a temperature of 

 about 39-5 C. (103 F.), or under a "broody" hen, first 

 marking each with the date. One or two should be 

 examined each day or oftener for the first four or five days 

 of incubation. Expose the embryo as directed on p. 576, 

 using warm normal (075 per cent.) salt solution (tempera- 

 ture as above), in order that after the first day the beating 

 of the heart and the circulation of the blood may not be 



