424 



TUNICATA. 



sac (Figs. 211 G', 212 A, a and b). The first differentiation in the 

 embryo consists in the appearance of a continuous superficial cell- 

 layer, in which we recognise the ectoderm of the embryo (Fig. 212 

 A, ec).* Important changes in the brood-sac take place at the same 

 time, its outer lamella, which represents a layer continuous with the 

 epithelium of the atrial-cavity of the parent (Fig. 212, a) soon chang- 

 ing into an extremely thin pavement-epithelium. The inner lamella 

 of the brood-sac (Fig. 212, b) represents the modified epithelium of 

 the oviduct and the follicle. In it we can distinguish a simple cell- 

 layer which at first completely encircles the embryo. This is the 

 inner lamella of the brood-sac in the strict sense of the term, and is 



R 



Flu. 212. Two sections through embryos of Sal pa democratica-mucfonata 



matic, after SALENSKY). A, younger stage; /), sagittal section through an older 

 stage, a, outer lamella of the brood-sac ; b, inner lamella of the same ; ec, ecto- 

 derm [layer of kalymmocytes, ectoderm not yet formed, KOROTNEFF] ; i, inner cell- 

 mass (entoderm, mesoderm ?) ; /, rudiment of elaeoblast; //<, rnesoderm(?) ; n. 

 rudiment of the nerve-centre ; p, remains of the follicle = rudiment of the placental 

 tissue ; x, ectodermal thickening, from which the covering of the placenta is derived 

 [this is follicular, i.e., maternal in origin according to KOROTNEFF]. 



no doubt derived from the modified oviduct. To the base of the sac 

 formed by the inner lamella is attached a cell -accumulation (p) which 

 probably represents the modified cell-material of the follicle ; this 

 forms the first rudiment of the placenta. [The placenta, including 

 the basal plate, according to KOROTNEFF (No. XXa.) is wholly 

 maternal in origin.] 



* [According to KOROTNEFF (No. XVIII.) the embryo becomes covered in the 

 first instance by a layer of kalymmocytes ; these SALENSKY mistook for the 

 ectoderm. The latter layer forms later by a rupture in the layer of kalym- 

 mocytes covering the embryo and a discharge of blastomeres through the gap 

 into the follicular cavity, where they become arranged as a layer, the future 

 ectoderm between the follicle and the embryo. Mesenchyme-cells are also dis- 

 charged into this cavity, and arrange themselves under the ectoderm. ED.] 



