548 THE ANATOMY OF INVERTEBRATED ANIMALS. 



occupied by a nucleated protoplasmic mass. Further on, this 

 mass becomes differentiated into an axile cord of protoplasmic 

 substance — the rhacJiis — and peripheral masses, each contain- 

 ing a nucleus and connected by a stalk with the rhachis, 

 which are the developing ova. Still further on, in the ovi- 

 ducal portion of the tube, the ova become free ; while, in tlie 

 uterine portion, they are impregnated, and acquire a hard, 

 often ornamented, shell. 



The testis is, generally, a single c^cal tube, in the blind 

 end of which cells are developed, much in ihe same way as in 

 the ovary : they become free in that part of the tube which 

 plays the part of a vas deferens. Contrary to what happens 

 in most animals, these spermatozoa retain the character of 

 cells, and may even exhibit amoeboid movements. The defer- 

 ential end of the testicular tube opens into a sac close to the 

 anus, from the dorsal wall of which one or two curved chiti- 

 nous spicula are developed. These are introduced into the 

 vulva of the female when copulation takes place, and appear 

 to distend it, in order to allow of the free passage of the sem- 

 inal corpuscles into the vagina, and thence into the uterus. 

 In the female organs, the seminal cells undergo further 

 changes, and eventually enter into, and coalesce with, the 

 substance of the ova. 



Yelk-division follows impregnation. The oval morula be- 

 comes indented on one side, and the embryo, as it grows, 

 folds itself in accordance with this indentation. In most, it 

 would appear that the central cells of the solid morula are 

 differentiated from the rest to form the endoderm, which thus 

 arises by delamination. But Biitschli ^ has recently shown 

 that the morula, which results from the division of the vitellus 

 of CiicuUaniis elegans, has the form of a flattened plate, com- 

 posed of two layers of blastomeres, the blastocoele being re- 

 duced to a mere fissure. The lamellar blastoderm next be- 

 comes concave on one side, convex on the other, and passes 

 into the gastrula form. The blastopore, at first very wide, 

 gradually narrows, and appears to be converted into the oral 

 opening of the worm. The mesoblast takes its origin from 

 certain cells of the hypoblast, which lie close to the mouth, 

 and grow thence toward the caudal extremity. The resem- 

 blance of this developmental process to that of Lumbricus is 

 obvious. 



1 " Zur EntwiclvelunTSorescluclite des Cucullanus elef/anft.''^ (ZelUohrift fi'ir 

 WISH. Zoolor/ie, 1876.) Hallez (" Revue des Sciences Naturelles," 1877) has ob- 

 served a similar process in Anguilltila aceti, but he denies that the blastopore 

 becomes the mouth. 



