CRANGON VULGARIS. 119 



In Crangon, so far as I have been able to see, all the 

 amoeboicl cells reach the surface and take part in the forma- 

 tion of the blastoderm before the process of gastruhition be- 

 gins. In that form no yolk pyramids occur. In Astacus, 

 Lereboullet ('62)? Bobretzky ('73) and Reichenbach ('77 

 and '86) have shown that they do occur, and Reichenbach 

 shows that they terminate in a central mass the nature of 

 which is doubtful. In Atyephyra (Ishikawa, '85) the yolk 

 pyramids are less evident, but in Palaemon (Bobretzky 

 »73) they are almost as piain as in Astacus, although 

 here the central mass is absent. In both Pal^emon and 

 Astacus, as well as in Eupagurus, all of the protoplasm 

 (certainly the nuclei) is used in forming the blastoderm, 

 unless the central mass have a nuclear or a protoplasmic 

 nature. In Atyephyra, on the otherhand, Ishikawa figures 

 numerous amoeboid cells remaining behind in the yolk 

 after the blastoderm is formed and when the process of 

 gastrulation has begun. 



The process of gastrulation in the decapods is so evident 

 that there is no difficulty in connection with it. The in- 

 vaginated entoderm may either contain a lumen (archen- 

 teron) as in Astacus, or it may be solid as in Palaemon, 

 Atyephyra, Crangon, and Eupagurus. In the former case 

 the deutoplasm is between the entoderm and the meso- 

 derm and ectoderm ; in the latter the entoderm cells form 

 a more or less complete wall around the yolk, so that this 

 substance comes to occupy the potential archenteric cavity. 

 There is but little to be said on these points beyond what 

 has been already said by other writers and hence no further 

 discussion is to be «:iven here. Amons: the other arthro- 

 pods however there are some points concerning which there 

 is a difference in interpretation and hence these may receive 

 some lischt from the conditions occurrino: in Cran2:on and its 

 allies. The confusion which has existed is my only excuse 

 for the foUovving excursus. 



