1 8 . r BIOLOGY OF THE LABORATORY MOUSE 



This bulge is the beginning of the posterior amniotic fold* (Fig. lo) . In like 

 manner the lateral wings of mesoderm, progressing around the egg cylinder 

 toward its anterior margin, give rise to folds along the sides of the cylinder. 

 These are the lateral amniotic folds. Finally, when the mesoderm reaches 

 the anterior margin of the egg cylinder, a small anterior fold is produced 

 (Fig. 12). The posterior, lateral and anterior folds should be thought of, 

 not as separate structures, but as a continuous constriction about the middle 

 of the egg cylinder which is drawn tighter and tighter as the folds develop. 

 Because of the very precocious development of the posterior amniotic 

 fold as compared with the anterior one, the constriction is eccentric, the 

 point of final closure being far towards the anterior margin of the egg 

 cylinder, t 



Before the anterior fold forms, small cavities J begin to appear in the 

 mesoderm of the posterior and lateral folds. These soon coalesce to form a 

 single large cavity, the extra-embryonic coelom, or, more concisely, the 

 exocoelom (Fig. 12). The exocoelom at this stage, and at all future stages, 

 is lined by mesoderm. For a short time a second cavity is present in the 

 posterior amniotic fold between the mesoderm and ectoderm (Fig. 12), but 

 this is a transitory structure of no particular significance. 



In less than a day after the first appearance of the amniotic folds, the 

 girdle which they form has closed. For a time a vertical strand of cells 

 adjacent and usually attached to the inner anterior wall of the exocoelom 

 marks the point of closure, but this soon disappears and the separation is 

 complete. The resulting condition is shown in the sagittal section repro- 

 duced in Fig. 13. 



Three cavities § are now present in the egg cylinder in place of the single 

 proamniotic cavity which it formerly contained. The most ventral of these 



* In Fig. 10 there may be seen a second fold pushing into the proamniotic cavity 

 just dorsal to the posterior amniotic fold. Sobotta (62) shows this in his Fig. 5, but 

 interprets it as an artifact. Our material would indicate that it is regularly though 

 briefly present. Its significance is unknown, but it is perhaps indicative of the very 

 rapid growth that occurs in the whole posterior wall of the egg cylinder at the time of 

 mesoderm formation. 



t In the rat, the anterior amniotic fold is much better developed than in the mouse, 

 and the constriction, therefore, less eccentric (26). 



I None of the embryos in our collection show this early stage in the formation of 

 the exocoelom. This description is based on the observations of Jolly and Ferester- 

 Tadie (26). 



§ In some cases, also, a transitory fourth cavity, the cavity between ectoderm and 

 mesoderm mentioned on page 18, second paragraph, is present. 



