DEVELOPMENT 52 I 



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 Pallenc, 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 

 AiniiiotJiea, 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 w'hat (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. Nymphon 

 hrevicollum (Hoek), the free larva is from the first provided with its 

 full complement of limbs. Certain other species of Nym^plion hatch 

 out in possession of four or five pairs of limbs, but in the great 



' Semper came near to discovering the fact when he saw, at Heligoland, ripe 

 eggs in a Plioxidiilidium that was, nevertheless, totally destitute of ovigerous 

 legs. The animal, he says, was adult and sexually mature : " Trotzdem fehlen 

 dem Tiere die Eiertrager vollstandig ; es muss sich also das Tier nocli mindestens 

 ein Mai hjiuten vor der Eierablag, und dabei miissen die Eiertrager gebildet 

 werden" {Arh. Inst. JFurzhtuy, 1874, p. 273). 



