xliv INTRODUCTION. 



all observers, that in the progressive growth of the ovum, 

 the embryo of the Amphipoda is rolled within the egg in 

 an opposite manner from that of the Isopoda. The latter 

 is folded backwards, so that the ventral appendages are 

 developed on the external surface, whereas the Amphipoda 

 is bent on itself, the ventral appendages being developed 

 on the inner surface. Dr. Fritz Miiller states that, in 

 Tanais, one of our aberrant genera, the development of 

 the larva is after the manner of the Amphipoda, and not 

 of the Isopoda, among which it is classified. 



The length of time between the epoch of the deposi- 

 tion of the ovum in the incubatory pouch, and the period 

 of the emancipation of the young animal from the care of 

 the parent, is probably about six weeks. We have ob- 

 served that to be the time required in the genus Asellus. 



At first the egg is perfectly round. It shortly after- 

 wards increases in one direction, becoming also somewhat 

 larger in Amphipoda at one extremity. Indistinct seg- 

 ments are now observable. The wall of the ovum is of 

 an elastic character, and yields to the movement of the 

 internal embryo. 



Probably about the middle of the period of incubation 

 the embryo quits the egg, for we have constantly taken it 

 from the pouch in a very immature condition, without 

 being enclosed in the egg-case. The larva at this period 

 is very immature, and enclosed within a general tunic, 

 which, without having any apparent vital connection with 

 the animal more than the original egg-case had, adapts 

 itself in general form to the whole creature, and fulfils 

 the duty of a protective tissue. As the embryo increases 

 in dimensions and completeness of form, so the tunic cor- 

 responds in size and form. At length, freeing itself from 

 this case, the larva strengthens in its own development, 

 but does not immediately quit the care of the parent. 



