XXI DEVELOPMENT 521 
before, and by some even after his time, the two sexes were 
constantly confused. 
Segmentation is complete, symmetrical in the forms with 
smaller eggs, unequal in those burdened with a preponderance 
of yolk (Morgan). In Pallene, as in the Spider’s egg, what is 
described as at first a total segmentation passes into a superficial 
or centrolecithal one by the migration outwards of the nuclei 
and the breaking down of the inner ends of the wedge-shaped 
segmentation-cells. The blastoderm so formed becomes con- 
centrated at the germinal pole of the egg. A thickened portion 
of the blastoderm (which Morgan compares to the “cumulus 
primitivus” of the Spider’s egg) forms an apparently blastoporal 
invagination (though Morgan calls it the stomodaeum), and from 
its sides are budded off the mesodermal bands. Meisenheimer 
has recently given a minute account of the early development of 
Ammothea, a form with small yolkless eggs. Here certain cells 
of the uniform and almost solid blastosphere grow inwards till 
their nuclei arrange themselves in an inner layer of what (so far 
as they are concerned) is a typical gastrula, but without any 
central cavity. The inner layer subsequently, but slowly, differ- 
entiates into the mid-gut, and into dorsal and lateral offshoots, 
the sources of the heart and of the muscles and connective tissues 
respectively. The further development of the egg takes place, 
as is usual in Arthropods, by the appearance, in a longitudinal 
strip or germ-band which enwraps the yolk, of paired thickenings 
which represent the cerebral and post-oral ganglia, and of others 
from which arise the limbs. Of these latter, the chelophores are 
the first to appear, on either side of the mouth; in Pallene the 
fourth pair appears next in order, followed by the fifth and sixth, 
and by the third and seventh just before the hatching out of the 
embryo; the second is lacking in this particular genus. Thus 
in Pallene (Dohrn, Morgan), and in some others, e.g. Mymphon 
brevicollum (Hoek), the free larva is from the first provided with its 
full complement of limbs. Certain other species of Myimphon hatch 
out in possession of four or five pairs of limbs, but in the great 
1 Semper came near to discovering the fact when he saw, at Heligoland, ripe 
eggs in a Phowichilidium that was, nevertheless, totally destitute of ovigerous 
legs. The animal, he says, was adult and sexually mature: “‘ Trotzdem fehlen 
dem Tiere die Kiertriger vollstindig ; es muss sich also das Tier noch mindestens 
ein Mal hiuten vor der Eierablag, und dabei miissen die Kiertrager gebildet 
werden” (Arb. Inst. Wiirzburg, 1874, p. 273). 
