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 rhachis — 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 the 

 uterine portion, they are impreg'nated, and acquire a hard, 

 often ornamented, shell. 



The testis is, generally, a single caecal tube, in the blind 

 end of which cells are developed, much in the 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 tlience into the uterus. 

 In the female ora-ans, the seminal cells underoro further 

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

 substance of the ova. 



Yelk-division follows impregnation. The oval morula bo- 

 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 B'dtschli * has recently shown 

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

 of GucuUaniis 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 frbm 

 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 Entwickelunsrsorescluclite des Cucnllanus eJegans.'^'* {ZeiUclirift filr 

 wiss. Zoologie^ 1876.) Hallez (" Rovue dcs Sciences Naturelles," 1877) Has'ob- 

 served a similar process in Anguillula aceti^ but he denies that the blastopore 

 becomes the mouth. 



