

\i:i.i>iiMi:\ i 01 MM I:\IM:\\I. POEM <>! TIM: BOOT, 197 

 which neural tube, ch<>rda. and primitive >e^ments are developed, 10 



that the cephalic and caudal rlevati>ns become joimd by meai 



a concave line. The ventral side of tin- trunk region, <>n the con- 



traiy, is greatly swollen and bulges otd \entrally ami laterally like 



a hernia. >ince it is tilled \\ith yolk cells. This swelling is therefore 



Called the ///;// 



Iii the further pro^n ss of development the embryo continually 

 acquire- a more lish-like shape. The .-interior and the poM>ii<>r 

 ends of the body, especially the latter, innvase greatly in length, 

 and the middle of the trunk becoim s thinner, for with the conMimp- 

 tion of the yolk-material the yolk-sac becomes smaller and finally 

 disappears altogether, its walls 1>; in incorporated into the ventral 

 wall of the intestine and that of the body. 



The interferences in the norm"/ c<'rse of development becom 

 in the same ratio as the yolk increases in amount, as it does in tin' 

 case of the meroblastic eggs of Fishes, fieptiles, an>l ]lir<ls. \\'ith 

 the latter the yolk is no longer broken up into a mass of yolk < 

 M in tht 1 case of the Amphibia; it participates in the process of 

 rli-ava^e, but only to a slight extent, inasmuch as nuclei make tin ir 

 way into the layer of yolk which is adjacent to the germ, and. 

 rounded by protoplasm, continue to increase in number by di\: 

 The gastrula-form is altered until it becomes unrecognisable; onlv 

 a small part, of its dorsal surface consists of cells, which are 

 arranged into the two primary germ-layers, whereas the whole 

 ventral side, where in the Amphibia the yolk-cells are found, i> an 

 nnsegmented yolk-mass. 



Thus we acquire in the case of the Vertebrates mentioned a 

 peculiar condition ; the embryo, if we regard the yolk as not 

 belonging to the body, appears to be developed from layers that are 

 spread out flat instead of from a cup like structure (Plate I., fig. 1, 

 213). Moreover we see even a greater distinction effected 

 In -tween the dorsal and ventral surfaces of the egg during develop- 

 ment than was the ca>e with the Amphibians. The fundaments of 

 all important organs, the nervous system, the chorda, the primitive 

 segments (Plate L, figs. 2, 8), are at first produced exclusively on the 

 former, whereas on the ventral side few and unimportant changes only 

 are to be observed. These consist principally in the extension of tin- 

 germ-layers, which spread out farther vent rally, grow over the yolk- 

 maxs (Plate I., figs. 2-5), and form around it a el- 



\ eral layers. This circumcrescence of the unsegmented yolk by 

 the germ-lay, i - i> accomplished, on the whole, very slowly, the more 



