XX EXPLANATION OF THE PLATES. 



h. Two diagrams illustrating the foctaJ cellular elements of tlie 

 placenta! tufts. These are the internal membrane, and the in- 

 ternal cells of the tufts, and along with the loops of umbilical 

 blood-vessels constitute the true foetal portion of the placenta. 



THE TESTIS AND ITS SECRETION IN THE DECAPODOUS 

 CRUSTACEANS. PLATES VII. VIII. 



PLATE VII. page 429. 



Fig. 1. Figures of Entozoa from the tubuli seminiferi of Orchestia 

 littoralis, probably allied to filaria, and supposed by M. 

 Kolliker to be the spermatozoa. This opinion, however, 

 is incorrect, as may be seen in the accompanying draw- 

 ings, where figures are given representing all the details 

 of the development of the true* spermatozoa. These are all 

 produced from cells, whereas the entozoa under considera- 

 tion are never seen within cells, but are in all cases generally 

 seen floating free in the seminal vessels. These filaria have 

 only been seen, so far as I am aware, in Amphipoda and 

 Isopoda. If they are spermatozoa, they must be produced 

 from cells ; and from what has been stated in the text, it 

 will be seen that in all the Crustacea, these cells, before pro- 

 ducing the spermatozoa, undergo several metamorphoses ; 

 and that the final changes take place in the spermatheca of 

 the female, where the seminal animalcules are produced. In 

 Amphipoda and Isopoda, where these supposed filaria exist, 

 we always find them high up in the testicle, and not occa- 

 sionally, but in great numbers. In the tertiary seminal cells 

 also, which are floating about among them, not the slightest 

 vestige of the worm can be observed. I am inclined to sup- 

 pose, therefore, that these thread-like worms, supposed by 

 Kolliker to be spermatozoa, are only parasites. 



Fig. 2. Representation of a primary germinal cell projecting from the 

 wall of the seminal tube. It has just burst, and the young 

 secondary cells are escaping and descending the tube ; during 

 the descent they increase in size, from their nucleus throwing 

 off nucleoli, the latter forming the tertiary generation. In 

 this figure it will be observed that the cell-walls of the 

 parent are quite smooth and unbroken, so that in ail proba- 

 bility the young arise from that portion of the cell attached 

 to the seminal tube. 



Fig. 3 Is a small quantity of the fluid from the spermatheca of the 

 female crab, showing the tertiary or spermatozoal cells after 

 they have burst from the secondary. As described in the 

 text, the spermatheca appears to be the organ in which the 

 seminal fluid undergoes the final and essential change which 

 fits it for impregnation. 



