510 SUMMARY OF CUERENT RESEARCHES RELATING TO 



corpuscles, sometimes surrounded by a fine granular substance filling 

 the whole of the segmentation cavity ; the important question is, are 

 these elements, which are clearly the first rudiments of the mesoderm, 

 derived from the endo- or the ectoderm ? Kowalevsky is in favour of 

 the latter view, while Metschnikoff holds to their endodermal origin. 

 If the larvcC are treated with acetic acid and immediately examined 

 evidence will be afibrded as to the presence of the first mesodermic 

 elements at a time when the ovum is still segmenting ; and, indeed, 

 indications of them were in two cases seen, where the developing ova 

 consisted of only eight blastomeres, for there is in them a central 

 corpuscle which appears to have a mesodermal significance. The 

 author has no distinct opinion as to the origin of this cell, but inclines 

 to doubt the explanation given by Metschnikoff. As to the still 

 earlier stages it is stated that the fecundated ova are developed outside 

 the body of the parent, but that they remain attached to the branchiae 

 for a certain time. After the appearance of four blastomeres two 

 divide, and so give rise to a six-stage of two large, and four smaller 

 cells. 



Development of Sipunculus nudus.* — B. Hatschek here gives 

 us another of his interesting and suggestive papers ; in the earlier 

 portion he deals with his observations of the phenomena of the 

 development of S. nudus at different periods, and then proceeds to 

 some theoretical considerations. 



Comparing tbe development of Sipunculus with that of Phas- 

 colosoma, he finds a striking resemblance between the two, this 

 point, however, being excepted, that in Phascolosoma there appear to 

 be no embryonic investments. If the observations of those who have 

 studied this form are correct, it follows that we have presented to us 

 the problem of whether the investments in question have been com- 

 paratively rapidly acquired by Sipunculus, or whether they have been 

 lost by Phascolosoma ; the latter form must be re-examined with 

 especial attention to this question. 



Hatschek believes that the Echiuridae completely agree with the 

 Annelida in their development, and, in discussing the relation of 

 Sipunculus to the Annelids, he brings this point into prominence. We 

 must ask ourselves how far the differences between Sipunculus and 

 the Annelids are of fundamental importance. If we take the mode 

 of formation of the germinal layer and of the closure of the blasto- 

 pore, we find a considerable agreement ; in the formation of the 

 gastrula, the development of the mesoderm from two marginal cells 

 of the primary endoderm, the cleavage of the mesodermal bauds into 

 a visceral and a parietal layer, and further, the mode of closure of the 

 blastopore and of the development of the oesophagus at the point at 

 which is placed the most anterior remnant of that orifice, the two 

 " types " present the same processes. Differences between the two are 

 to be found in the remarkable development of embryonic investments 

 in Sipunculus, but, at the same time, these seem to have appeared only 

 after it separated from the Annelid-stock. Compared with the 



* Arbeit. Zool. Inst. Wien, v. (1883) pp. 61-140 (G pis.). 



