32 Dynamic Theory 



trula. The animal consists of a double skin in the shape of a bag or 

 sac. The outside skin is the ectoderm and the inside or lining is called 

 the entoderm. It propagates by means of ova or female cells impreg- 

 nated by male or sperm cells. 



FIG. 47. 







FIG. 47. Egg cleavage and gastrulation of the ascidian (sea squirt), a. Parent egg 

 (cytula). The natural size is from 1-10 to 1-5 of a millimetre in diameter. In the inte- 

 rior is the nucleus 1-50 of a mm. in diameter enclosing the nucleolus. 6. Cleavage into 

 4 cells, c. Germ vesicle or Madder formed after repeated cleavage now called bias- 

 tula, d The gastrula ( bell gastrula), formed from the blastula by invagination. 



Hseckel. 



In some cases both kinds of sexual cells are developed from the ecto- 

 derm cells. In other cases the sperm cell is an ectoderm cell and the 

 ovum is from the entoderm. Many of * these gastread animals have 

 some adventitious or incidental appendage of more or less advantage to 

 them, as cilia or hairs and stems or stalks, by which they are fastened 

 to some object. But the vital and essential part of the animal when 

 mature is nothing more nor less than a gastrula of the bell type. The 

 embryos of the following animal varieties, as given by Haeckel, pass 

 through this form of the bell gastrula, viz: (a) most of the plant ani- 

 mals, as low sponges, medusae, and. corals, hydrapolypes; (b) many low 

 worms (sagitta) phoronis, ascidia, many nematodes; (c) a few mollusca 

 terebratula, argiope, pisidium; (d) most star animals (echinoderma) ; (e) 

 a few low articulates (some branchiopods, copepods, tardigrades) ; (/) 

 skulless vertebrates (amphioxus). 



The above list contains no animals of a high order. There is in it 

 only one vertebrate and that the lowest of all, the amphioxus. Since 

 they are not viviparous it is clear that they could scarcely survive 

 through a gastrula form, which depends upon nourishment from with- 

 out, unless they lived in a fluid element. Therefore, they are all in- 

 habitants of water or other fluid, or are parasites in moist parts of other 



