THE EVIDENCE FROM DEVELOPMENT. 



179 



unmistakable affinities and likenesses to the development of the 

 sea-squirts and of other invertebrate animals. Noting the absolute 

 likenesses which exist between the development of the sea-squirt and 

 lancelet, there seems every justification for the scientific belief that 

 both animals have arisen from a common stock. The gulf between 

 Vertebrate and Invertebrate life in this view, no longer exists ; and 

 the lancelet may be legitimately regarded as the parental form of all 

 Vertebrates, from the fish to man. 



Still higher in the vertebrate scale do our researches in develop- 

 ment lead us when we approach the study of the chick and its early 

 life-history. And what is true of the chick's development is, with 

 greater or less modification of details, true of the production of every 



FIG. 03. DEVELOPMENT OF VERTEBRATE. 



other vertebrate animal, man included. In the developing egg of the 

 bird, the yolk undergoes segmentation (Fig. 93, A, B, c, D), as in the 

 sponge, ascidian, and lancelet; but the process is partial in the 

 bird, whilst it affects the entire egg-mass in the development of lower 

 life. The blastoderm is duly formed as the result of segmentation, 

 and from this substance 

 seen in the cicatricula, or scar 

 of the egg arises the future 

 fowl ; the great mass of the 

 yolk and white serving as 

 nutrient material for the de- 

 veloping bird. Soon, the 

 cells of which the blastoderm 

 is composed are seen to form 

 themselves in three layers 

 (Fig. 94, E, M, H) : an outer layer or epiblast\ a middle layer or 



N 2 



FIG. 94. 

 SECTION THROUGH A DEVELOPING VERTEBRATE. 



