392 GENERATION. 



wall is the inner layer of the same membrane. The two 

 round orifices ({) are sections of the Wolffian ducts. 1 



The figures we have just described, it must be borne ia 

 mind, represent transverse sections of the body of the chick, 

 made through the middle portion of the abdomen. In our 

 explanations of these figures, we have not adhered absolutely 

 to the text of Briicke, but have made use of the very elegant 

 semi-diagrammatic illustrations by Waldeyer, whose explana- 

 tions are remarkably clear and satisfactory. 2 Our explana- 

 tions, however, particularly those of Fig. 39, are sufficiently 

 extended to enable us to study the development of special 

 organs. The posterior parts, it is seen, are developed first, 

 the situation of the vertebral column being marked soon after 

 the enclosure of the neural canal by the vertebral plates; 

 and, at about the same time, the two aortse make their ap- 

 pearance, with the first traces of the pleuro-peritoneal cavity. 

 The next organs in the order of development, after the vas- 

 cular system, are the Wolffian bodies, which are so large and 

 important in the early life of the embryon. The intestinal 

 canal is then a simple groove, and the embryon is entirely 

 open in front. Were we now to follow the process of devel- 

 opment farther, we should see that the visceral plates advance 

 and close over the abdominal cavity, as the medullary plates 

 have closed over the neural canal. Thus there would be formed 

 a closed tube, the intestine, lined by the thin, internal blasto- 



1 A careful study of the process of development in the chick, as described by 

 Eeichert, an account which is still followed by some recent writers of authority 

 (LONGET, Traite de physiologie, Paris, 1869, tome iii., p. 880, et seq.\ shows that 

 the division of the blastodermic layers which we have adopted, as the one most 

 nearly in accordance with the latest and most reliable observations, differs very 

 little, in its essential features, from that proposed by this eminent German em- 

 bryologist. Reichert accurately described the intermediate membrane, from 

 which the greatest part of the embryon is developed, and regarded what we have 

 termed the external and the internal blastodermic layers as of comparatively 

 little importance. (REICHERT, Das Entwickelungsleben im Wirbelthier-Reich, 

 Berlin, 1840, S. 102, et seq.) 



2 WALDEYER, Eierstock und Ei, Leipzig, 1870, Taf. iv. 



