62 SUMMARY OF CURRENT RESEARCHES RELATING TO 



MoUuscoida. 



o. Tunicata. 



Morphology of Tunicata.* — MM. E. van BeBeden and C. JuHn have 

 continued their researches on the morphology of Tunicates. After an 

 historical review of the state of opinion in regard to some of the main 

 morphological problems, the authors proceed to give a detailed account 

 of the embryonic development of Clavellina Bissoana. 



I. The chief conclusions of the first chapter are as follows : — (1) The 

 first sedimentation plane corresponds to the median plane of the gastrula. 

 The entire right half of the body arises from the right blastomere. The 

 median organs, such as medullary tube and notochord, arise from a double 

 rudiment, forming two identical portions, separated by the median plane. 

 In the earliest stages no cells are exactly median. Afterwards, however, 

 this is not so, and some cells may pass from one side to the other. (2) The 

 second plane is transversal, dividing each blastomere into anterior and pos- 

 terior portions ; the third is horizontal. (3) The formation of the ectoderm 

 results from successive protrusions from mixed or undifferentiated elements. 

 After 32 are thus formed the process stops, the endoderni is formed, and 

 the invao'ination begins. (4) The last ectoderm cells formed as above are 

 always disposed at the periphery of the anterior ectodermic plate, and since 

 the marcin of this plate forms the rudiment of the nervous system, it may 

 be affirmed that the cells forming the epidermis are formed before those 

 which give rise to the nervous system. (5) At the 8 stage the tubular and 

 vertical segmentation cavity opens to the exterior at each end ; at the 16 

 statue it is closed, and with the formation of 32 cells it disappears. It has 

 therefore no genetic connection with spaces subsequently occupied by the 

 mesenchyme. (6) When gastrulation begins, the medullary cells which 

 form the outline of the nervous system, are readily distinguishable. They 

 form a ring immediately surrounding the blastopore, and markedly larger 

 in front, than on the side of, or behind, the latter. (7) At the same stage 

 the common outline of notochord and mesoderm is seen to be separated 

 from the rest of the endoderm which forms the gut. The former also 

 appears as a ring round the blastopore. (8) An important portion of the 

 medullary plate lies behind the blastopore, forming part of its posterior lip 

 and contributing to form the arch of the medullary tube. Kowalewsky's 

 results are here confirmed. The process of closure is throughout essen- 

 tially the same. (9) At no position or stage of development is the medul- 

 lary plate separated from the epidermis to form the floor of a canal, of 

 which the roof is formed from the epidermis. (10) The nervous system 

 primitively consists of two exactly similar portions, but these come to be 

 intimately connected, and to take up in part a median position. (11) The 

 notochord developes at the expense of the primitive endoderm below the 

 median and problastoporic portion of the medullary plate. It arises in 

 front of the blastopore at the expense of a portion of the rudiment which 

 also forms the mesoderm. It arises as a furrow and forms a cord by 

 the progressive approximation of the margins. It only becomes secondarily 

 median. (12) The mesoderm developes at the expense of the endoderm, in 

 the form of two lateral portions separated dorsally by the notochord, and 

 vcntrally by the gut-forming endoderm. An anterior portion is resolved 

 into mesenchyme, the posterior region forms the caudal muscles. The first 

 portion arises in enterocoelous fashion in the form of lateral diverticula 

 from the arcbenteron. The second portion arises similarly, but this pos- 



* Arcb. dc- Biol., vi. (1885) pp. 2.37-47G (9 pis.). 



