DEVELOPMENT OF LIMULUS POLYPHEMUS. 161 



by Prof. E. Van Beneden, have by their clear views, based on a profound study of the 

 earliest embryonic changes, afforded me the greatest aid in these studies. He has shown 

 that in Sacculina, one of the Rhizocephala ; in the Copepoda (Anchorella and Lerneopoda) ; 

 in Gammarus, Asellus, Caprella, Nebalia and Crangon, and in the Acarina (the ''deutovum" 

 of Claparede), "the blastodermic cells secrete a first cuticular membrane." This "blasto- 

 dermic cuticle" is secreted just after the blastodermic cells appear, at the time when the 

 egg contains a protoplasmic mass charged with nutritive elements, which hold in suspension 

 some cellular nuclei. In reality it is formed of a certain number of embryonic cellules 

 recognizable by their nuclei, but not so by their contents. " It is at this moment that there 

 is formed around the vitelline mass a very fine anhystic {anhyste) membrane, which is in our 

 opinion a primitive {premier) embryonic moult ; it is probably secreted by these primitive 

 cells, and represents in some degree a first embryonic moult." This is not, as Claparede 

 thought, the homologue of the "larval membrane" of Ligia and Mysis. Van Beneden is 

 quite positive that this latter membrane no more than the nauplian skin, present in the 

 eggs of all Crustacea, has nothing in common with the amnion of insects. And in this I 

 would fully agree, on the evidence he presents, with the Belgian embryologist. I could not 

 detect any such membrane in Limulus ; the embryonic cellular membrane (protoderm) 

 moulted (and not secreted) after the primitive disk has been formed, is directly homologous 

 with the "amnion" of insects and the scorpions (it has not been observed in the spiders). 



So far as I am aware Apus ^ is the only other crustacean in which a protoderm like that 

 of Limulus is formed. Dohrn^ remarks that cells are visible in the "amnion" of Oniscidne 

 and Idotea. It is not comparable with the nauplian or larval skin, as it is composed of 

 distinct cells and is entirely circular, with no lateral diverticula surrounding the appendages. 

 Why should this layer of blastodermic cells be thus thrown off and serve as a tough, dense 

 protection to the embryo, even late in embryonic life, serving, instead of the chorion which 

 splits apart and falls off, as a temporary egg shell ; while in most other Crustacea, the 

 blastoderm cells are absorbed ? I think the reason is a biological one, and that the presence 

 of this membrane is not of particular taxonomic importance. The eggs of Limulus are 

 laid loose in the sand and remain there or are washed about by the waves during a month or 

 six weeks, and possibly over a whole season. As the embryo increases in size, the tough, 

 dense, inelastic chorion splits asunder rather than stretches, the dilatable protoderm enlarges, 

 becomes dense and acts as a vicarious chorion. Zaddach^ has described the same bursting 

 of the chorion and the presence of what he calls a delicate internal membrane, which is in 

 fact a protoderm, in Apus cancriformis. 



n know of nothing in the eggs of any other crustacean ex- tlie embryo. Zaddach {Be Apodis cancriformis Analome et 



cept Apus, comparable to this pecuhar, inner egg membrane. Historia Evolutionis, pi. iv, figs. 1, 2, 3), represents the 



In the eggs taken from the ovisac of Apus longicaudatus Le- splitting of the chorion, and the embryo remaining in the 



conte, and before the embryo has begun to develop, there is inner, tender and pellucid membrane. 



a similar membrane. It is very thin, free from, but lying -Morphology of the Arthropoda, Humphrey's Journ. 



next to, the structureless chorion. It consists of a single Anat. and Phys., 18G7. 



layer of irregular cells, varying in size, and often with a dis- ^De Apodis cancriformis, etc., pi. iv, fig. 3. Zaddach calls 



tinct nucleus. In a profile view of the uninjured eggs these this " membrana interna," remarking that it exists in the 



cells are seen to be rounded and project out as in the inner ovary, and is afterwards accommodated to the body of the 



egg-membrane of Limulus. As in Limulus, it does not as- embryo. He does not notice its cellular structure. That 



sist in building up the embryo, and as in that genus, it is this membrane is the same as the internal membrane formed 



evidently a protective membrane, and brought into requisi- in the ovary seems to us very doubtful, as we have endeav- 



tion at the time of the splitting and falling off of the chorion, ored to show that it is like the amnion of Limulus. 

 to take its place, while it increases in size with the gi'owth of 



MliMOIUS BOST. SOO. NAT. HIST. VOL. II. 41 



