PERICARDIUM, PLEUROPERITONEUM, DIAPHRAGM AND MESENTERIES. 385 



simplicity of the mesentery, concurrent with corresponding simplicity in the 

 loops and coils of the intestine. In this anomaly the intestine has failed to 

 arrive at its usual complicated condition and the mesentery has not undergone 

 the usual processes of folding and fusion (p. 382 et seq.}. Minor variations in 

 the mesenteries and omenta are probably due largely to imperfect fusion of 

 certain parts with one another and with the body wall. It is not uncommon to 

 find the ascending or descending colon, or both, more or less free and mov- 

 able. This condition is due to imperfect fusion of the mesocolon with the body 

 wall (p. 383). If the greater omentum is wholly or partially divided into sheets 

 of tissue, the two primary lamellae have failed to fuse completely (p. 381). 

 This divided condition is normal in many Mammals. 



PRACTICAL SUGGESTIONS. 



The Ccelom and Common Mesentery. Chick embryos afford excellent material for the 

 study of the formation of the ccelom and common mesentery. Use successive stages during 

 the latter part of the first day and during the second day of incubation. Remove the 

 embryo from the egg, being careful to keep it as nearly as possible in the natural position, 

 fix in Flemming's or in Zenker's fluid, cut transversely in paraffin and stain with Heidenhain's 

 haematoxylin. Time can be saved by staining in Mo in borax carmin, but the differentia- 

 tion is not so clear. The various stages in the development of the coelom and mesentery 

 are well shown. If serial sections are cut, one can often trace the successive stages in one 

 embryo by examining sections in succession through the series, for the bending of the germ 

 layers begins in the head region and gradually progresses toward the tail. 



The Primitive Pericardial Cavity. The dilatation of the ccelom in the cervical region to 

 form the primitive pericardia! cavity can be seen in transverse sections of chick embryos of 

 the latter part of the first day of incubation. Prepare the specimens as directed in the pre- 

 ceding paragraph. The specimens prepared for the study of the ccelom and mesentery 

 will serve this purpose if sections from the cervical region are selected. 



The Septum Transversum. The early stages of the septum can be seen in sections of 

 chick embryos of the second day of incubation, prepared as directed above. It is best to cut 

 serial sections. By tracing the omphalomesenteric veins, the ridges formed by the veins 

 can be seen projecting into the ccelom. If later stages are examined, the ridges will be seen 

 to extend across the ccelom and to fuse with the ventro -lateral part of the body wall. The 

 ridges form the anlage of the septum. 



Later Stages. The study of the later stages becomes more difficult as the structures 

 increase in complexity. Chick or mammalian embryos in different stages are fixed in 

 Zenker's fluid or Bouin's fluid, sectioned transversely in paraffin, and stained with Weigert's 

 haematoxylin and eosin. It is best to cut serial sections, and time can be saved by staining 

 in toto with borax-carmin if the embryos are not too large (not more than 10 mm.). 



While much can be learned by examining a series of sections, it is advisable to recon- 

 struct from serial sections (see Appendix) the developing organs in one or two stages. 

 The models thus obtained are extremely useful in understanding the relations of the 

 structures. 



After the embryo has attained a considerable size, very careful gross dissections will often 

 prove instructive. 



