46o NATURAL SCIENCE. June. 



Here is a natural parallel, with the remarkable difference that not 

 many embryos, but only a single embryo comes from the separate 

 pieces, which, after pursuing for a time a separate course, re-unite. 

 But more astonishing matter remains. The actual embryo is first 

 blocked out in follicle-cells, 



" These form layers and undergo foldings and other changes 

 which result in an outline or model of all the general features in the 

 organisation of the embryo. While this is going on, the development 

 of the blastomeres is retarded, so that they are carried into their 

 final positions in the embryo while in a very rudimentary condition. 

 Finally, when they have reached the places which they are to occupy, 

 they undergo rapid multiplication and growth, and build up the 

 tissues of the body directly, while the scaffolding of follicle-cells is 

 torn down and used up as food for the true embryonic cells." 



" An imaginary illustration may help to make the subject clear. 

 Suppose that while carpenters are building a house out of wood that 

 brickmakers pile clay on the boards while they are carried past, and 

 shape the lumps of clay into bricks as they find them scattered 

 through the building where they have been carried with the boards. 

 Now, as the house of wood approaches completion, imagine that 

 brickmakers build a brick house over the wooden framework, not 

 from the bottom upwards, but here and there wherever the bricks are 

 to be found, and that as fast as parts of the brick house are finished 

 the wooden one is torn down. To make the analogy complete, 

 however, we must imagine that all the structure which is removed is 

 assimilated by the bricks, and is thus turned into the substance of 

 new bricks to carry on the construction." 



__^_^_ Fig. 5 shows a stage where the 



,'^ 7"/^ cells resulting from the early 



"^ ^ divisions of the egg-cell have been 



^ '■■-',, separated by the intruding follicle- 



cells. Figures showing the outlining 

 "^ of the embryo by the follicle- 

 ^ cells would be too complicated 

 ^ M to reproduce without elaborate 



'^^_ & \_r # explanation and colouring, as in the 



^■*!:'" '"' - beautiful illustrations of Dr. Brooks's 



"^ monograph. But it must be re- 



"-i^_^ =- peated that, down to details like the 



"^"^" -■—-'- structure of the nervous system, this 



FiG.5-Thesegmentationspheres system of preformation in follicle- 



lying in a mass oi tolhcle-cells that ■'n '^ rr^, r , r n .1 



have grown in between them. ^ells OCCUrs. The fate of all the 



structures formed of follicle-cells is 

 the same. They degenerate, forming food for the true embryonic cells. 

 Former observers had described something like this. Salensky, for in- 

 stance, stated that the embryo of Salpa began as a true embryo, but 

 afterwards the accessions from the follicle wall turned the embryo 

 into a bud. He believed, therefore, that its development was to be 

 compared in no way with ordinary embryological processes. Dr. 

 Brooks considers that, however abnormal, it is a true embryo, and the 

 appearance of budding is illusory. Certainly his figures bear out this 

 view completely. 



Dr. Brooks believes that comparative embryology has established 

 that the ancestors of all the Tunicates once possessed small eggs with 

 little or no food-yolk, and that these small eggs divided completely, 

 giving rise first to a mulberry-shaped mass and then to a hollow 

 sphere of cells. By the inpushing of the cells at one point they gave 



-_!^;f . 



