TYPE OF THE PHYLACTOLAEMATOUS LARVA. 



33 



(No. 48), and Jullien (No. 47a), and more recently by Davenport 

 (No. 46a), Kraepelin (No. 50), and Braem (No. 45a).* 



The ovary (Fig. 15 ov) is an aciniform growth of the inner meso- 

 dermal layer of the body-wall, lying on the oral side of the polypide, 

 immediately below the so-called duplicating bands (posterior parieto- 

 vaginal muscles of Allman) and above the last daughter-bud. 

 Single mesodermal cells increase in size, and become changed into 

 young egg-cells, while other mesoderm-cells become grouped round 

 them as the follicular epithelium. One embryo only develops in 

 each individual (Keinhard, Kraepelin). Fertilisation and the 

 commencement of cleavage take place within the egg follicle. Later, 

 however, the young embryo passes into a peculiar brood-sac (ooecium. 

 Fig. 15 x, 16 o), which functions as a uterus, and in which the 

 further embryonic development takes place. The ooecium, when it first 

 appears (Fig. 15 x), closely resembles a young 

 polyp-bud, being a pocket-like invagination of 

 the bilaminar body- wall. The outer or meso- 

 dermal layer of this bilaminar invagination 

 is destined to envelop the embryo, while the 

 inner ectodermal layer undergoes degeneration 

 in the course of further development. It 

 was formerly somewhat doubtful whether 

 the ooecium was altogether distinct from the 

 primary egg-follicle. According to Reinhard, 

 it was derived directly from the epithelium 

 of the ovary. Whereas, according to 

 Metschnikoff, the egg passed out of the 

 original follicle into the body- cavity, where 

 the ooecium grew round it secondarily, 



according to Kraepelin and Braem, the 



Fig. 15.— Section through a 

 portion of the zooecial wall 

 of Plumatellafungosa (after 

 Braem). ec, ectoderm ; at, 

 mesoderm ; ov, ovary ; x, 

 rudiment of the ooecium. 



rudiment of the ooecium presses closely on 



the egg while still in the ovary, and receives it from the egg- follicle. 

 According to the statements of most authors, cleavage is quite 

 regular. Kraepelin, however, thought that he found certain 

 differences in the blastomeres in the first stages. A cleavage-cavity 

 early appears within the embryo, and a coeloblastula (Fig. 16, A, e) 

 develops, which, by a solid ingrowth of cells at one pole (Fig. 16, B), 

 passes into the gastrula stage. The point of ingrowth, according to 



* [The form in which the development of the larva and of the polypide has 

 been best worked out is Plumatella {Alcyonclla) fungosa (=P. polymorpha, 

 Kraepelin).— Ed.] 



