828 REPORT—1890. 
throughout all the earlier stages the optic lobes are, as in other vertebrates, on 
the dorsal surface, and only shift down to the sides shortly before the time of 
hatching. 
Crabs differ markedly from their allies, the lobsters, in the small size and rudi- 
mentary condition of their abdomen or ‘tail.’ Development, however, affords 
abundant evidence of the descent of crabs from macrurous ancestors, for a young 
crab at what is termed the Megalopa stage has the abdomen as large as a lobster 
or prawn at the same stage. 
Molluscs afford excellent illustrations of recapitulation. The typical gastropod 
has a large spirally-coiled shell; the limpet, however, has a large conical sbell, 
which in the adult gives no sign of spiral twisting, although the structure of the 
animal shows clearly its affinity to forms with spiral shells, Development solves 
the riddle at once, telling us that in its early stages the limpet embryo has a spiral 
shell, which is lost on the formation, subsequently, of the conical shell of the 
adult. 
Recapitulation is not confined to the higher groups of animals, and the 
Protozoa themselves yield most instructive examples, A very striking case is that 
of Orbitolites, one of the most complex of the porcellanous Foraminifera, in 
which each individual during its own growth and development passes through the 
series of stages by which the cyclical or discoidal type of shell was derived from 
the simpler spiral form. 
In Orbitolites tenuissima, as Dr. Carpenter has shown,!' ‘the whole transition 
is actually presented during the successive stages of its growth. For it begins life 
as a Cornuspira,... . its shell forming a continuous spiral tube, with slight 
interruptions at the points at which its successive extensions commence; while its 
sarcodic body consists of a continuous coil with slight constrictions at intervals. 
The second stage consists in the opening out of its spire, and the division of its 
cavity at regular intervals by transverse septa, traversed by separate pores, exactly 
asin Peneroplis. The third stage is marked by the subdivision of the “ peneropline ” 
chambers into chamberlets, as in the early forms of Orbiculina. And the fourth 
consists in the exchange of the spiral for the cyclical plan of growth, which is 
characteristic of Orbitolites; a circular disc of progressively increasing diameter 
being formed by the addition of successive annular zones around the entire peri- 
hery.’ 
; The shells both of Foraminifera and of Mollusca afford peculiarly instructive 
examples for the study of recapitulation. As growth of the shell is effected by the 
addition of new shelly matter to the part already existing, the older parts of the shell 
are retained, often unaltered, in the adult; and in favourable cases, as in Orbitolites 
tenuissima, all the stages of development can be determined by simple inspection 
of the adult shell. 
It is important to remember that the Recapitulation Theory, if valid, must 
apply not. merely in a general way to the development of the animal body, but 
must hold good with regard to the formation of each organ or system, and with 
regard to the later equally with the earlier phases of development. 
Of individual organs, the brain of birds has been already cited. The formation 
of the vertebrate liver as a diverticulum from the alimentary canal, which is at 
first simple, but by the folding of its walls becomes greatly complicated, is another 
good example; as is also the development of the vomer in Amphibians as a series 
of toothed plates, equivalent morphologically to the placoid scales of fishes, which 
are at first separate, but later on fuse together and lose the greater number of 
their teeth. 
Concerning recapitulation in the later phases of development and in the adult 
animal, the mode of renewal of the nails or of the epidermis generally is a good 
example, each cell commencing its existence in an indifferent form in the deeper 
layers of the epidermis, and gradually acquiring the adult peculiarities as it 
approaches the surface, through removal of the cells lying above it. 
1 -W. B. Carpenter, ‘On an Abyssal Type of the Genus Orbitolites, Phil, Trans. 
1883, part li. p. 5&3. 
