292 



dermi viventi si possono dividere in 2 grandi gruppi, di cui uno com- 

 prende le Oloturie ed i Crinoidi , l'altro le Ophiure^ le Asterie e gli 

 Echini. Adottando la nomenclature proposta daHaeckel'- chia- 

 merò il 1° gruppo dei Monorcho^iia, il 2*^ gruppo dei Pentorchonia. 

 sebbene ad essi io dia altra estensione ed altro significato, come lo di- 

 mostra il differente posto da me assegnato ai Crinoidi^ che daHaeckel 

 sono stati ascritti ai Pentorchonia. 



Nel lavoro completo darò più ampie spiegazioni sulle divergenze 

 che in proposito esistono tra i miei studi, fondati sulla ontogenia. e 

 quelli, fondati sull' Anatomia comparata e sulla Paleontologia dal 

 geniale Zoologo di Jena. 



Cagliari, Istituto d'Anatomia comparata e Zoologia, 20 Maggio 1899. 



3. The External Features in the Development of Lepidoslren 

 paradoxa, Fìtz. 



By J. Graham Kerr. Read May 4, 1899. Royal Society of London. (Abstract.) 



The paper opens with a short account of the habits qî hepidosiren 

 as observed in the Gran Chaco. A description is then given of the 

 external features in the development. The more important points in 

 this may be summarised as follows. 



The egg is very large, 6,5 — 7 mm in diameter. It is surrounded 

 by a special capsule at first thick and almost jelly-like in appearance, 

 later on (after fertilisation) thin and horny. Outside this was found in 

 rare cases a thick jelly resembling that of the common frog's egg. 

 The egg is without a trace of dark pigment. Segmentation is com- 

 plete, resembling most nearly that of the <s,^;g of Amia, and leads to a 

 condition with an upper hemisphere of small cells with large segmenta- 

 tion cavity, and a lower of large yolk cells. Gastrulation begins with 

 the appearance of a row of depressions, or a continuous groove along 

 about one-third of the whole extent of the margin between small and 

 large cells. During its progress the small-celled portion spreads over 

 the lower yolk cells by the addition to its margin of small cells split 

 off from the yolk cells. As the groove referred to deepens into a slit 

 to form the archenteron, it becomes gradually shorter, and the eventual 

 complete blastopore is a crescentic slit only about a quarter of the 

 length of the original groove. The medullary folds soon appear runn- 

 ing forwards from the blastopore. There is no trace externally of a 

 blastoporic or protostomal seam running along the back between the 



12 Die Amphorideen und Cystoideen. Beiträge zur Morphologie und Phylo- 

 genie der Echinodermen. Festschrift zum siebenzigsten Geburtstage von Carl 

 Gegenbaur, am 21. August 1896. p. 160 e seg. 



