DEVELOPMENT 521 



before, and by some even after his time, the two sexes were 

 constantly confused. 1 



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 ;ip]e;irance, in a longitudinal 

 strip or germ-band which enwraps tin 1 yolk, of p.-iiivd thickenings 

 which represent the cerebral and post-oral gan-li;i, ;md of others 

 from which arise the limbs. Of these l.-iiter, the chelophores are 

 the first to appear, on either side of tin- mouth : in I'nlle,ir the 

 fourth pair appears next in order, followed by the fifth ;md sixth, 

 iind by the third and seventh just before the Imtrliin- out of thr 

 embryo; the second is lacking in this p;irtieul;i,r genus. Tims 

 in Pallene (Dohrn, Morgan), and in some others, e.g. .\W/'//// 

 Irevicollum (Hoek), the free l;ir\;i is from tin- first pn>\ ide<l \\ith its 

 full complement of limbs. ( Vrl;iin other species of .Y// ////<//// h;iteh 

 out in possession of four or li\e pniis of limbs, but. in the grejit 



1 Semper came near to discoverm- the I'aei \\lim lie saw, at Heligoland, ripe 

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

 legs. The animal, he says, was adult and sexually mature: ' Trot/don tVhlrii 

 dnn Tiere die Eiertriiger vollstiindig ; es muss sidi also das Tier noeh mindest. us 

 ein Mai hiiuten vor der Eicralila^, und daliri miisM'ii dii- Kieri ia<_ f er 

 werden" (Arb. lust. Wurzburg, 1874, p. 273). 



