2 
MR. E. RAY LANKESTER ON THE 
self-impregnation occurs is not definitely ascertained, but it seems possible. Leydig 
was unable to give a satisfactory account of the arrangement of the ducts and generative 
glands from a study of the large Cyclas. cornea , and I have not attempted to enter upon 
this question with the much smaller Pisidium. 
Plate 1. fig. 3 is drawn from an egg removed from the brood-pouch. It is now gra- 
nular in the body owing to the inception of deutoplasm, and has undergone impregnation. 
This is inferred from the fact that it is enclosed in a transparent envelope or egg-shell 
of membranous consistency. The nucleus and nucleolus seen now in the egg may or 
may not be the original germinal vesicle and spot of the egg-cell. Though these 
structures disappear in some eggs, it cannot be asserted that they may not be persistent 
in others. One point of interest in this and other eggs figured on the Plate is the 
presence of the membranous envelope to the egg, which was not found by either Leydig 
or Schmidt in their studies on Cyclas. 
This fact has been especially insisted upon with regard to Cyclas , and it is therefore 
important to note the presence of the envelope in Pisidium. It is very delicate, and is 
ruptured and discarded after the first stages of development. 
Plate 1 . fig. 15 shows a similar egg in its envelope ; in this egg two nucleoli are 
present in the nucleus (1 germinal spots and vesicle). 
Plate 1. fig. 16 gives a surface- view of the first pair of cleavage-grooves. They are 
seen to embrace the whole egg. 
Plate 1. fig. 17. The cleavage-grooves are now four meridional, and the first circum- 
ferential is beginning to make its appearance. These two views are taken so as to 
exhibit the grooved surface of the yelk. 
Plate 1. fig. 4 exhibits the four nuclei of the four first cleavage-segments. 
Plate 1. figs. 5 & 6. The cleavage-products have increased largely in number, so 
that the egg is now a ball of embryonic cells or a polyplast. 
Plate 1. figs. 7 & 8 exhibit a very important condition of the early development. 
A deep in-pushing of the surface of the polyplast is obvious, the result being the invagi- 
nation of a part of the superficial cells, in the same way as a woven nightcap is tucked 
in to make it assume the form of a cap. The occurrence of this primitive invagination 
of the embryonal polyplast has been demonstrated by Kowalevsky in Ascidia and 
Amjphioxus , and has now been recognized in some members of all the large groups of the 
animal kingdom. The process results in the production of a form which I proposed* 
* Annals and Magazine of Natural History, May 1873. 
The formation of a Gastrula by invagination is obviously indicated though not recognized by the author, in 
LoviN’s admirable studies on the development of Mollusca, as also in Karl Yogt’s memoir on the development 
of Actceon. In the Nudibranchs and in Limax lhave observed and drawn the Gastrula in course of formation 
by invagination (see Contribution No. III.). 
Dec. 1874. — Also in Lymnceus (see Quart. Journ. Mic. Sci., October 1874) and in Paluclina vivipara. 
March 7th, 1875. — Though the name Gastrula is expressive, I am at this moment inclined to prefer the 
original term Planula, on account of the ascribing of a mouth by Professor Haeckel to his typical Gastrula. 
The orifice of invagination, when it occurs, is not known to be a mouth. I propose to call it the blastopore. 
