380 E. RAY LANKESTER. 



which are important in modern embryology, and shows that 

 the orifice which he took for the first indication of the mouth 

 is what we now know as the orifice of invagination or blas- 

 topore which can be traced in subsequent development not 

 to the mouth, but to the anus. 



Stage 1. — From the early spring until the middle of 

 October large specimens of Paludina contain in the upper part 

 of the oviduct ova which are in the condition of simple un- 

 segmented corpuscles such as that represented in Plate XXV, 

 fig. 1. Each egg in Paludina is contained when in the oviduct, 

 in a relatively very large envelope distended by a thick but 

 transparent albuminous fluid in which the egg-corpuscle 

 floats and from which it cannot be entirely liberated. 



The egg-envelopes lying somewhat nearer the mouth of 

 the oviduct contain embryos in various stages of development, 

 so that it is possible from one specimen to obtain a very com- 

 plete series. The youngest oviducal eggs such as that repre- 

 sented in Plate XXV, fig. 1, present in the living condition no 

 indication of nucleus, of germinal vesicle or germinal spot. 

 The egg-protoplasm at its periphery appears to be hyaline 

 and highly refracting, whilst it is charged with fine granular 

 matter beneath the surface and with minute spherical 

 particles of a bright yellow colour which give the whole e^^ 

 an orange-brown tint. 



Stage 2. — The ova in the various stages of division may 

 be found in one or another specimen. I have selected for 

 fig. 2 an embryo in which the cells are still of large size. 

 They appear all to share equally the possession of the finely 

 granular and yellow coloured food material and to present no 

 grouping into a hemisphere of smaller and a hemisphere of 

 larger cells. Hence the " polyplast" or " morula" as Haeckel 

 has termed it, is of the simplest type — an " archimorula." 



Stage 3. — In fig. 3 an embryo is represented in which the 

 division of the embryonic cells has proceeded further, whilst 

 a central space or " cleavage cavity'^ has been formed. The 

 opacity of the embryo does not permit one to make out 

 very satisfactorily the size of this cavity, and whether the 

 spherical form is really possessed by the " blastula" stage as 

 this one-cell-layered vesicle is called in HaeckePs terminology. 

 Possibly a slight cup-shaped depression already extends over 

 one pole of the blastula. 



Stage 4. — In figs. 4 and 5 are reproduced sketches taken 

 from living specimens showing the formation of the endo- 

 derm by invagination. The ectodermal cells are seen to be 

 somewhat columnar and but little charged with granular 

 matter, though they retain to a late period some of the 



