398 COMPARATIVE ANATOMY chap. 



by a specially large nucleus. This divides, and as the primitive sexual cell produces 

 the rudiments of the germ glands. 



A blastomere in contact with this sexual cell yields the endoderm, and the 

 blastomeres, which surround the primitive sexual cell, and the primitive endoderm 

 cell, yield the Mesoderm (except the sexual organs and nervous system). In a 

 later stage (Fig. 269, A) we find 4 genital cells, 32 endoderm cells, and 12 mesoderm 

 cells ; all the remaining blastomeres represent the ectoderm. First the 12 mesoderm 

 cells sink down, then the endodermal plate becomes invaginated [B). In place 

 of the gastrula mouth, which probably closes, the definitive mouth appears later. 

 The genital cells increasing in number to 8, then sink down ((7). Nutritive yolk 

 remains in the segmentation cavity. Two paired frontal groups of cells appear as 

 rudiments of the neural plate. Later on this plate consists of several layers and 

 yields anteriorly the brain, and posteriorly the retina. The cesophageal commissures 

 and the ventral chord arise in situ as thickenings of the ectoderm. The mesoderm 

 spreads itself out on the inner side of the ectoderm, on the surface of the rudiments 

 of the genital glands, and around the mid-gut which is at first solid : this mid-gut 

 alone proceeds from the endodermal invagination. The shell gland is of mesodermal 

 origin and only opens secondarily at the point of the 2d maxilla. The compound 

 eye is from the first paired. 



II. Cetochilus septentrionalis (Order, Copepoda). 



The segmentation is total and yields a blastula with a small segmentation cavity. 

 Under the blastomeres at a certain stage 2 symmetrically placed cells can be 

 distinguished as primitive mesoderm cells and a few others also symmetrically 

 placed as endodermal cells. The dividing mesoderm cells sink deeper inwards. In 

 still later stages we can recognise at the posterior end of the mesoderm, on each side, 

 a large primitive mesodermal cell. The endoderm also becomes invaginated into the 

 segmentation cavity. The gastrula mouth closes in a median line fi'om before back- 

 ward. The dividing mesoderm cells fill the segmentation cavity. The stomodseum 

 and proctodseum arise by means of ectodermal invaginations. At a later {Nauplius) 

 stage there arises at each side behind the Naitplius eye an ectodermal thickening 

 connected with the brain, which severs itself from the integument but degenerates 

 later. These are to be considered as the rudiments of the paired compound lateral 

 eyes with their optic ganglia, which are wanting in most Copepoda. A pair of meso- 

 derm cells lying on the ventral side of the intestine represent the rudiments of the 

 sexual glands. The 2 cells move higher on the 2 sides of the intestine, and become 

 surrounded with smaller mesoderm cells, which yield the rudiments of the ducts, 

 which at first are solid. In the 1st Cetochilus stage the paired genital rudiments 

 fuse to form an unpaired dorsal sexual gland. The heart develops out of a paired 

 rudiment of mesoderm cells. 



III. Branchipus {Phyllopoda). 



In the hatched Nauplius larva under the cuticle, the segments of the first maxillce 

 and the first two trunk segments, with their limbs, have already begun to form. An 

 elongated portion follows, in which the segmentation of the mesoderm streaks 

 has begun. This does not reach as far back as into the anal segment. The two 

 mesoderm streaks unite posteriorly directly in front of the anal segment to form a 

 ventral plate which fonns a budding zone. Its cells rapidly increase, and as the 

 development of the larva progresses new mesoderm segments continually become 

 demarcated from the budding zone and spread out under the ectoderm to the dorsal 

 middle line. But this segmented germ streak merely represents the parietal layer 



