980 



SCIENCE. 



[N. S. Vol-. XV. No. 390. 



(c) 'Is there then no law of individual 

 development?' asks von Baer. He believes 

 there is and bases it on the following con- 

 siderations : 



The embryos of mammals, birds, lizards 

 and snakes present such similarities in their 

 entirety as well as in the development of 

 corresponding parts that except for differ- 

 ences in size it would be difficult to dis- 

 tinguish them. 



The further we go back in the history of 

 development, the greater do we find those 

 similarities, and only gradually do those 

 special characters emerge from the general 

 type which distinguishes the smaller groups 

 of animals. To this the history of the 

 chick in every stage of its development 

 bears witness. 



In the beginning, when the back 

 closes, it is a vertebrate and nothing more. 

 When the embryo becomes more and more 

 separated from the yolk; when the gill 

 clefts close and when the urinary sac 

 grows out, it becomes a vertebrate unsuited 

 for free life in the water. Only later, a 

 difference in the extremities is recognizable 

 and the bill appears; the lungs move up- 

 wards and the air sacs are established as 

 rudiments. Now there is no longer any 

 doubt that the form is a bird. While the 

 avian characteristics become augmented by 

 development of the wings and air sacs, 

 and by the fusion of the carpel cartilages, 

 the webs of the feet disappear and we have 

 a terrestrial bird. Later when the crop is 

 developed and the nasal scale appears, the 

 terrestrial bird takes on the characters of 

 the Gallincn and finally those of the domes- 

 tic fowl. 



{d) Briefly, we may say that the point 

 of greatest resemblance in the development 

 of two animals is remote in proportion to 

 the amount of difference they exliibit in 

 their adult condition. The differences be- 

 tween the long-tailed and the short-tailed 

 crabs are not very great. The crayfish has 



in the middle of its embryonic life a tail 

 short in proportion to the broad sternum, 

 and it is difficult to distinguish at this stage 

 from the short-tailed crabs, which, accord- 

 ing to Cavolini's figures, are in their em- 

 bryonic condition comparatively long- 

 tailed. The further we go back in the 

 history of development, the greater do we 

 find the similarity between the feet and 

 the organs of mastication. We have thus 

 not only an approach to the fundamental 

 type, but a resemblance to the Stomato- 

 poda, the Amphipoda and the Isopoda, 

 which in their fully developed state differ 

 more from the Decapoda than these do 

 among themselves. To this may be added 

 that in the Decapoda according to Rathke, 

 the heart appears spindle-shaped, and 

 many other points of similarity, so far un- 

 recognized, must exist. Still earlier, when 

 the feet are present as small laterally bud- 

 ding knobs, and tiie gills are not yet visible, 

 a resemblance with true insects in their 

 embryonic condition is not to be denied. 



These considerations bring us to the 

 question whether there is not, early in the 

 history of development, a stage in which 

 the embryos of vertebrates resemble those 

 of invertebrates. In another place, von 

 Baer shows that even the series of seg- 

 mented animals begins development with 

 a primitive streak, and that therefore dur- 

 ing this brief period there is a resemblance 

 between them and the early stages of ver- 

 tebrates. In the germ, all embryos devel- 

 oped from a true egg probably resemble 

 each other and in this lies a strong reason 

 for considering the germ as the animal 

 itself. 



(e) The further back we go in devel- 

 opment, the more points in common do we 

 find in very different animals, and so the 

 question arises whether in the beginning 

 all are not fundamentally alike and 

 whether there does not exist a common an- 

 cestor. All true eggs appear to have a 



