Lilian V. Sampson 489 



From the earliest stages of the series, the entire yolk is divided into 

 cells, which are larger in the center and smaller at the periphery (Fig. F) . 

 The cell walls are sometimes not clear in poorly preserved eggs of early 

 stages/ The cells are, in all stages, smallest in that part of the yolk where 

 the anterior and posterior parts of the gut are forming. In later stages 

 while the yolk is disappearing from the gut walls, it is lost also in the cells 

 of the middle yolk-mass immediately beneath the mesentery and by the 

 time the cavity of the alimentary canal is established in the central yolk- 

 mass, the wall beneath the mesentery is differentiated into a layer of small 

 cells that pass abruptly into huge yolk-laden cells several times their 

 own size (Fig. M, g, e, y). 



The frog may feed before the yolk is entirely absorbed, for the stomach 

 of a hatched frog contains green food, while part of the intestine is still 

 loaded with yolk.° 



3. Heart and Blood- Vessels. 



The details in the development of the blood-vessels have not been 

 worked out, but the chief events in their history have been determined. 



Heart. — The heart develops from the mesoderm ventral to the pharynx. 



In stage IV, it has arrived at the condition of a bent tube (Fig. I) with 

 a slight constriction between the auricular and the ventricular parts. 

 It undergoes the usual changes of further twisting, thickening of the 

 ventricular walls, etc. The longitudinal septum in the truncus appears 

 at about stage VII, but the septum between the auricles, which in the 

 frog appears simultaneously with the septum of the truncus, is not 

 found in Hylodes until stage IX ; it attains no great size until XI, after 

 which it separates a very small left auricle from the right. The relatively 

 late development of this septum may be correlated with the slow growth 

 of the lungs and consequently of the pulmonary circulation (cf. p. 493). 



Veins. — For a full understanding of the vitelline circulation, the ex- 

 amination of living material is essential. From the preserved prepar- 

 ations no constant arrangement of vessels on the yolk can be determined. 

 In stage IV, the yolk is literally covered with great vessels, in which are 

 large blood corpuscles filled with yolk granules ; the vitelline blood is col- 

 lected in a large vein, which passes through the liver, and constitutes in 

 early stages the posterior end of the heart (Fig. Gl, vv) . After stage VI, 



* It is stated that the yolk-mass of Desmognathus fusca and of Ichthyophis 

 is at one period segmented only peripherally, and that in Plethoden cinereus 

 also the boundaries of the yolk-cells disappear. 



"The embryo of a viviparous salamander will feed although yolk is still 

 contained in the digestive tract. 

 35 



