BEING A WORM 



191 



sperm 



...mesooCarm. 



mesoderm 



onus 

 V. 



segments 9 and 10, it receives sperm cells from the other worm that 

 have been stored there. The girdle is passed down over the anterior 

 end of the worm, slipped off, forming a closed case which contains the 

 eggs, sperms, and a nutritive fluid. These capsules may be found in 

 late spring under stones, 

 boards, logs, or in manure 

 heaps. After fertilization, 

 the egg of the earthworm 

 divides first into two, then 

 four, then eight cells, and 

 so on, continuing until a 

 hollow ball of cells, called a 

 hlastula, is formed. These 

 cells are not all the same 

 size, larger cells appearing 

 on the lower pole of the 

 sphere, which begins to 

 flatten and show a depres- 

 sion, forming eventually 

 a hollow cuplike affair, 

 called the gastrula. This 

 process known as gastrula- 

 tion places the larger cells 

 of the lower pole on the in- 

 side of the cup where they 

 become the endoderm, 

 leaving the outer cells of 

 the sphere to form the 

 ectoderm. Meantime a 

 third layer of cells which 

 lies between the other two 

 layers buds off and be- 

 comes the mesoderm. This latter layer gives rise to the musculature, 

 blood vessels, and most of the excretory and reproductive tissues ; the 

 endoderm forms the food tube and much of the glandular material con- 

 nected with it ; the ectoderm gives rise to the epiderms, the nervous 

 system and sense organs, and the outer portions of the nephridia, repro- 

 ductive ducts, and digestive tracts. The young worms remain in th(^ 

 egg case until they are about an inch in length. When first hatched 

 they have no clitellum, since this organ appears only in mature worms. 



gostrola 



Stages in development of earthworm. Fig- 

 ures II-V. Segmentation of egg and formation 

 of blastula. Figures VI-VIII. Sections, show- 

 ing formation of mesoderm as a band of cells. 

 IX. Late stage of gastrula, showing coelomic 

 spaces in mesoderm bands. X. Longitudinal 

 section of young worm showing food tube, mouth 

 and anus. (After Sedgwick and Wilson.) 



