396 SUMMARY OF CURRENT RESEARCHES RELATING TO 



blood. The pcrigonadial spaces (so-called generative glands) are also 

 ccelornic in character, and in Ccphalopods and tho archaic Neomenia they 

 are continuous with the pericardium. In both Molluscs and Arthropods 

 the ancestral blood-vessels havo swollen and enlarged, so as to form 

 largo irrogular spaces which havo blocked up and so obliterated tho 

 jn-eviously existing ccelom. In the Arthropoda the ccelom is represented 

 by the tubular generative gland, and as a system of small spaces (lymph- 

 spaces) in the connective tissue of Astacus and Limulus, and as the 

 internal terminal vesicle of the green glands and other ncphridia present 

 in various Arthropoda. The heart and pericardium of the Arthropoda are 

 absolutely peculiar to the group, and characteristic of all its members. 



Prof. Lankcster considers that each pair of valvular apertures in 

 the heart of an Arthropod represents a pair of distinct tubular veins, 

 which in the ancestral form brought blood to the heart from the gills. 

 These veins havo dilated, and their adjacent walls have been absorbed, 

 so that we now have, instead of a series of veins, a great continuous 

 blood-series on each side of the heart or dorsal vessel. Capillaries of 

 the finest dimensions have been found in certain parts of Astacus and 

 Limulus ; between them and unconnected with them, in the connective 

 tissue, there is a system of spaces containing a coagulable fluid ; into 

 this system the tubular nephridium, which becomes the coxal gland of 

 Limulus, opens, so that these are remnants of the ccelom, elsewhere 

 blocked up and obliterated by the swollen veins which form the haemocoel. 

 The tubular generative glands of Arthropods are to be explained as 

 perigonadial ccelom communicating with the exterior through modified 

 nephridia. 



Mollusca. 

 a. Cephalopoda. 



Homology of Germinal Layers of Cephalopods.* — Mr. S. Watase 

 has a preliminary communication on the formation of the germinal layers 

 in Loligo Pealii and other Cephalopods. A noticeable feature in the 

 distribution of the food-yolk and germinal protoplasm is that the line of 

 demarcation between them is perfectly distinct from the beginning of 

 the embryonic history, so that segmentation goes on regularly without 

 any disturbing effect on the food-yolk. 



In the stage which is regarded as the gastrula stage, the germinal disc 

 consists of three zones — the central, which is a circular area of single 

 cells ; the intermediate, two-cell-thick, zone ; and the marginal single- 

 cell zone ; some of the cells of the intermediate zone are spindle-shaped, 

 and so show the characteristics of the future yolk-membrane cells. 

 This germinal disc may be regarded as an inverted gastrula with its 

 concave side turned towards the yolk-mass ; the line of junction between 

 the marginal zone and the food-yolk corresponds to the lip of the 

 blastopore, and the shallow cavity to the archenteron. Looked at in 

 this way, the first origin of yolk-membrane cells near the inner side of 

 the marginal zone fully justifies its comparison with the lining of the 

 gastrula cavity or to the hypoblast. The delamination of the interme- 

 diate zone before th e completion of the lining of the gastrula cavity must 

 be looked upon as precocious development due to the influence of food- 

 yolk. 



It is clear, then, that the author recognizes in the early stages of 



* Johns.Hopkins Univ. Circ, vii. (1888) pp. 33-4. 



