ZOOLOGY AND BOTANY^ MICROSCOPY, ETC. 183 



completely lodged. This process becomes more and more marked, 

 and the egg begins to exhibit a segmentation cavity. Two grooves 

 appear, and divide the cul-de-sac into three portions ; as they deepen, 

 the sac gets the form of an irregular mass, and the median portion 

 gives rise to the peripheral layer (placental membrane, Todaro) of the 

 placenta ; in the next stage the lower wall of the follicle increases in 

 size and gives rise to a mass of several rows of cells, which will go to 

 form part of the placenta. The fold formed from the inferior 

 divisions of the sac forms two layers of the circular fold, which is 

 destined to cover the whole of the embryo (caduca externa, Todaro). 

 During this process the superior division of the cul-de-sac has become 

 much more completely attached to the upper portion of the follicle. 

 The circular fold grows more and more over the embryo. 



The embryonic appendages are thus developed : the outer layer 

 of the embryo becomes applied to the inner face of the follicle, its 

 lower portion, with which there is connected the mass formed by the 

 internal layer, separates from this follicle, and so gives rise to the 

 placental cavity ; that portion of the outer layer which invests it is 

 the foetal placenta, the rest of the embryo forms the endoderm and 

 apparently the rudimentary ectoderm. A little later the young Salpa 

 becomes invested in a single layer which forms its skin, the endo- 

 dermic mass becomes completely detached from the foetal placenta, 

 and forms a nucleus around which the principal organs are developed. 

 The foetal placenta unites with the placental membrane to form the 

 complete placenta, a third layer in which is formed from the mass 

 of several rows of cells, already mentioned. The foetal placenta 

 increases in size and then undergoes a retrograde development ; thus 

 the structure of this part is simplified. The remaining stages are 

 simpler. 



There are, then, three parts concerned in the formation of the 

 embryo and its appendages ; two, the follicle and an expansion of the 

 wall of the branchial sac, are developed from the mother ; the thii-d is 

 formed from the egg. The upper portion of the primitive cul-de-sac 

 forms the outer wall of the primitive incubation-cavity, the fold at 

 the base bounds the definite uterine cavity, while the median portion 

 gives rise to the placental membrane. From the ovum the embryo 

 proper and the foetal membrane are developed. The author thinks that 

 the so-called placental membrane has really no placental function, but 

 rather serves to keep the incubation-sac in its place in the middle of 

 the great uterine cavity. Reduced to its simplest terms, we may say 

 that the mother furnishes two incubatory pouches, connected by a 

 membrane which maintains the first within the second pouch, and the 

 maternal placenta ; while to the embryo there is to be ascribed a 

 simple expansion, which, like the allantois of the Mammalia, is destined 

 to form the central portion of the placenta. 



Modifications of the Avicularia in Bryozoa.* — Mr. T. Hincks 

 considers that there can be no reasonable doubt that the vibraculum 

 is a derivative from the avicularium and not an independent modifica- 



* Ann. and Mag, Nat. Hist., ix. (1882) pp. 20-5 (4 figs.). 



