DECAPODA. 127 



-development should be further studied in Ntjssbaum's recent treatises (Nos. 30 

 and 31), Which, in the important points, agree with the description here given. 

 According to Nussbaum, the anterior cleavage -sphere produced by the first 

 division yields a cap of cells which entirely grows round the second cleavage- 

 sphere. When this circumcrescence is nearly completed, division of cells begins 

 actively in the superficial layer near the pointed pole of the egg, where a 

 gastrula-invagination now forms. The layer formed by this invagination, and 

 which is now turned inward, starting from the pointed pole on one side, grows 

 round the inner (food-yolk) sphere towards the blunt pole. " In the meantime 

 this inner sphere (or the second lower cleavage-sphere) has also divided, and 

 continues to divide ; but it is certain that the inner layer of cells of the gastrula 

 are not formed from the cells derived from the inner or lower cleavage-sphere." 

 According to Nussbaum, the anterior cleavage-sphere would thus yield not only 

 the ectoderm, but the entoderm as well. The opposition between his view and 

 that of Nassonow is, however, lessened, if we may assume that the superficial 

 layer of cells does not arise exclusively from the division of the anterior 

 cleavage-sphere, but that other elements derived by micromere-formation from 

 the posterior sphere also take part in it.* The latter would then play the same 

 part as the central mass of food-yolk in other Crustacean eggs that have super- 

 ficial cleavage. 



The distribution of the mesodermal elements in the later embryonic stages, as 

 well as in the young Nauplius stages of the Cirripedia appears to be irregular. 

 Grobben (No. 21), however, found in the posterior part of the body of Sacculina 

 and Balanus a few T large cells, evidently caught in the act of proliferating, 

 arranged on each side as a short mesoderm-band. 



D. Decapoda. 



The very primitive invagination-gastrula of Lucifer, which develops 

 from a coeloblastula, has already been described (p. 108) and figured 

 (Fig. 49 C — cf. Brooks, Nos. 42 and 43). Unfortunately the stages 

 which connect this gastrula with the Nauplius have not been 

 investigated. 



The development of the egg of Lucifer is characterised : (1) By the small 

 quantity of food-yolk which at first appears equally distributed in the blasto- 

 meres, (2) by the very regular course of the cleavage and the development 

 of a comparatively large blastocoele, (3) by the development of an invagination- 

 gastrula. The archenteron thus arising has at its apex four spheres rich in 

 food-yolk ; these spheres have been constricted off from the neighbouring 

 entoderiu-cells, and their significance is as yet undetermined. Brooks considers 

 them to be vestigial yolk-pyramids, which would correspond to the primary 

 pyramids of Astacus and Palaemon. The form which resembles Lucifer most 

 closely is perhaps Penaeus, inasmuch as the mid-gut here also develops direct 

 from the primary invagination— archenteron (Haeckel, No. 47). 



Among the other Decapoda, a comparatively primitive position 

 is occupied by Astacus, whose development has been made known 

 chiefly by the researches of Bobretzky (No. 41) and Reichenrach 



* [Sec Groom, App. to Lit., Cirripedia L, and footnote, p. 112]. 



