FUSION OF CAHDIAC AXLACJES IN THE CAT 49 



and the following dilatiition where endothelium and myocardium 

 laterale are closely approximated is interpreted as auricle. The 

 actual stages of fusion other than in the bulbs and the early 

 stages of the loops are not represented in Miss Parker's material. 



In the cat from the first the myoepicardial mantle has an ob- 

 liquely sagittal direction in consequence of which the retrocar- 

 diac plate broadens somewhat as it is followed caudad, and 

 tapers in the opposite direction, (embryos of 4-7 somites). The 

 topography of the parietal cavity in an embryo of 8 somites is 

 shown in figure 1, which agrees closely with Fleischmann's 

 figure of a total view of a cat embryo of this stage. ^ 



Before the infolding of the splanchnopleure is begun, the 

 topographj^ of the mantles is such as would seem to entail their 

 earlier approximation caudad where the retrocardiac plates are 

 broadest. The direct contrary is the case; cephalad where these 

 plates are narrow the bulbar segments are quickly brought into 

 apposition, while the ventricular portions diverge and are wide- 

 ly separated caudad, nor is there sufficient difference in the 

 width of the gut opposite these two segments of the heart to ac- 

 count for their difference in position. The problem thus pre- 

 sented is not easy of solution and its difficulty is increased by 

 the rapidity of the process, for the heart passes in the period re- 

 quired for the development of three somites from the position 

 shown in figure 1 to that in figure 3. Thus in the interval be- 

 tween the appearance of the eighth and eleventh somites the 

 formation of the foregut is completed as far as the atrial ex- 

 tremity of the heart, and the anlages of that organ have been 

 moved through a dorso-ventral arc of m arly 180° and become 

 approximated ventral to the pharynx. 



It is not possible on the basis of the material in hand to at- 

 tempt a complete solution of this problem for which several 

 processes extrinsic as well as intrinsic require minute investiga- 

 tion. Primarily there are the changes incident at this period 

 in the general shape of the region, notably the shortening, asso- 

 ciated with the beginning ventral flexion of the forebrain, which 



* Fleischmann, A. EmhryoloKische Untersufhunficii. I, 1889. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 29, NO. 1 



