344 



ANIMAL DEVELOPMENT 



Sense Organs, by means of which the activities of animals are 

 regulated and directed. 



The small egg-cell of the Lancelet passes out of the body, and 

 is fertilized in the surrounding- sea- water, after which it undergoes 

 cleavage to become a Blastula, which in this case is a hollow 

 sphere [blastosphere), the wall of which is formed by a single layer 

 of cells (fig. 863). These are rather larger on one side of the 

 sphere. The two-layered stage or Gastrula is then reached by a 

 sort of in-pushing of the larger cells to constitute the lining of 

 a large central digestive cavity (archenteron), that communicates 

 with the exterior by a mouth, technically known as the blastopoi^e. 

 This internal layer of cells is the eiidoderm, while the smaller 



Fig. 



-Young Embryos of Lancelet .^Amphioxits), enlarged, and bisected to show interior, a, Blasto- 

 sphere. B, Commencing gastruia. c, Gastrula. See te,\t. 



external cells are collectively known as the ectoderm. The two- 

 layered Gastrula next gives rise to a three-layered stage, in a way 

 which may be gathered by reference to fig. 864, which represents 

 cross-sections through four embryos of different age, the youngest 

 being on the left. The wall of the central digestive cavity bulges 

 out at the upper corners, as indicated by dark shading in the 

 figures, and the pouches thus formed gradually get pinched off, 

 coming to lie between the ectoderm and endoderm. They subse- 

 quently grow right round the body and constitute the mesoderm, 

 or middle embryonic layer. The cavities of the pouches become 

 the body-cavity of the adult, i.e. the lymph-containing space, or in 

 this case series of spaces between body-wall and the wall of the 

 dicfestive tube. 



It will be worth while to consider to what parts the three layers 

 of the embryo respectively give rise. i. The innermost layer 

 {endoderm^ becomes the thin cellular membrane {epitheliuni) which 

 lines most of the digestive tube and also its outgrowths, such as the 



